Version 2.1, December 31, 2002
Edited by David Sahnow
Including contributions from:
Helen Hart, Van Dixon, Bill Oegerle, Ed Murphy, Jerry Kriss and others
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The FUSE Instrument and Data Handbook
Version 2.1, December 31, 2002
Edited by David Sahnow
Including contributions from: |
Version 2.1 (December 31, 2002):
Version 2.0 Major changes from Ver. 1.1 (February 4, 2000 version):
This handbook describes the FUSE instrument and the science data produced by the FUSE Science Data Processing Pipeline System. It includes descriptions of the data acquisition, data processing, file naming conventions, and data formats. Methods and suggestions for displaying and interpreting the data products are also given. Finally, known instrumental artifacts and their effects on the data are described.
Figure 2.1-1. Optical layout of FUSE instrument showing 4 channel design.
This multi-channel design allowed the coatings on the mirrors and gratings to be selected to maximize reflectivity in the wavelength ranges above and below ~1020 Å. Two mirrors and two gratings are coated with SiC to provide wavelength coverage below ~1000 Å, while the other two mirrors and gratings are coated with aluminum and a LiF overcoat. The Al+LiF coating provides about twice the reflectivity of SiC at wavelengths > 1050 Å, but has very little reflectivity below 1020 Å. We will thus refer to "the SiC channels" and the "LiF channels" below.
The four channels can be thought of as comprising two nearly identical "sides" of the instrument. A side consists of one LiF and one SiC channel, each of which produces a spectrum that falls onto a single detector. Each channel has a bandpass of about 200 Å. Thus, at least two channels are required to cover the entire ~290 Å wavelength range of the instrument. All four channels cover the 1015-1075 Å region.
Figure 2.1-1 also shows the orientation of the instrument prime
coordinate system (X,Y,Z). The two LiF channels are on the +X side of
the instrument, which is always kept in the shade (i.e., the -X side always
points toward the sun). This orientation minimizes the amount of
sunlight that can make its way down the baffles surrounding the LiF
channels. Minimizing stray light in the LiF channels is crucial to the
operation of the Fine Error Sensor (FES) guidance camera, which operates
at visible wavelengths. The orientation of the satellite is actually biased by
several degrees in roll around the Z axis in order to keep the radiator
of the operational FES in the shade.
2.2 Telescope Mirrors
The four telescope mirrors are identical off-axis paraboloids. The blanks are made of Zerodur, with a triangular rib structure on the back to provide a lightweight but very stiff substrate. Each mirror is attached to the front of a honeycomb-sandwich intermediate plate by means of tangential-blade flexures, which minimize mounting-induced distortions. Three stepper-motor actuators are attached to the rear of the intermediate plate, which allow tip-tilt-focus adjustment of the mirror. Refocusing is required periodically as the structure shrinks due to desorption of water from graphite epoxy. The tip-tilt mechanism is used to provide rough alignment of the mirrors to the FPA entrance apertures.
The off-axis angle of the SiC mirrors and the LiF mirrors differ slightly. This angle is defined by aperture stops placed over the surfaces of the mirrors. Otherwise, the mirrors are identical except for the coatings. Some of the specifications of the mirror assemblies are given in Table 2.2-1. The point spread function (PSF) at the focal plane places ~90% of the light within a diameter of 1.5 arcsec.
Thermal control is maintained by heaters attached to the intermediate plates, which radiatively couple to the rear of the mirror substrates.
| Mirror type | Off axis parabola |
| Substrate material | Zerodur |
| Size of clear aperture | 387 × 352 mm |
| Focal length | 2245 mm |
| Off axis angle (to optical center) | 5.3668 deg (SiC mirrors) 5.4678 deg (LiF mirrors) |
| Coatings | 2 mirrors with ion-beam-deposited SiC; 2 mirrors with LiF over aluminum |
The four apertures include:
| Aperture | Keyword | Dimensions (arcsec) | Comments |
|---|---|---|---|
| high resolution | HIRS | 1.25 × 20 | - |
| high throughput | MDRS | 4.0 × 20 | - |
| large square | LWRS | 30 × 30 | Nominal Aperture |
| pinhole | PINH | ~0.5 (diameter) | Not Used |
The geometrical arrangement of the apertures, drawn to scale, is shown in Figure 2.3-1.
Figure 2.3-1. The locations of the FUSE apertures in the sky for a slit position angle of 0° with North in the +Y direction. Positive aperture position angles correspond to a counter-clockwise rotation of the apertures on the sky.
Each FPA can be moved independently in two directions. Motion tangential to the Rowland circle, which is roughly in the dispersion direction and perpendicular to the apertures, allows co-alignment of the channels and permits "focal plane splits" for high signal/noise ratio observations of bright targets. Motion in "Z" (perpendicular to the plane of the FPA mirror) enables focusing of the apertures with respect to the spectrograph grating and detector.
The front surface of the FPAs on the two LiF channels are
reflective in visible light. Light not passing through the apertures is
reflected into a visible light CCD camera. Images of stars in
the field of view (FOV) around the apertures are used for acquisition and
guiding by this camera system, called the Fine Error Sensor (FES).
Further information on the FES is described
below.
2.4 Spectrograph
The spectrograph has a Rowland circle design, with a diameter of 1652 mm. Light entering the spectrograph through an FPA aperture illuminates one of four diffraction gratings. The gratings are holographically ruled on spherical substrates made of fused silica. Table 2.4-1 lists some of the design parameters of the spectrograph and diffraction gratings.
| Property | SiC | LiF |
|---|---|---|
| Rowland circle diameter | 1652 mm | 1652 mm |
| Ruling density at grating center | 5767/mm | 5350/mm |
| Grating angle (alpha) | 24.0° | 25.0° |
| Grating angle (beta) | 9.31660 deg @ 986 Å | 9.76612deg @ 1107 Å |
| Grating dimensions | 266 mm (dispersion) × 275 mm (spatial) | |
| Grating type | first generation, type II holographic | |
FUSE can obtain spectra from about 905 Å to 1187 Å. The spectra from the four channels are imaged onto two microchannel plate detectors, with a SiC spectrum and LiF spectrum on each (from each aperture). Therefore, each detector covers the entire wavelength range. The two channels are offset on the detector perpendicular to the dispersion direction to prevent the spectra from overlapping. Each detector is divided into two functionally independent segments (A and B) separated by a small gap. To ensure that the gaps do not fall at the same wavelength region in both detectors, they are offset slightly with respect to each other in the dispersion direction. The wavelength coverage of each of the eight detector segment/channel combinations is listed in the FUSE Observer's Guide. Nearly the entire wavelength range is covered by more than one channel, and the important 1015-1075 Å range is covered by all four.
Although the holographic rulings allow for partial correction of astigmatic optical aberrations, the image of a point source still has a vertical extent of 150 to 1100 µm (~ 14 to 100 arcsec) on the detector, depending on wavelength. This means that virtually no spatial imaging capability is available, since the HIRS and MDRS apertures are only 20 arcsec in length (which projects to 220 µm on the detector).
The SiC channels have a dispersive plate scale of 1.03 Å/mm while the LiF channels have a scale of 1.12 Å/mm. Coupled with the size of the detector pixels, this results in a scale of 6.2 mÅ/pixel in the LiF channel and 6.7 mÅ/pixel in the SiC channel (in the X or dispersion direction).
The FUSE White Paper
The FUSE Wavelength Calibration describes the details of the pipeline's
wavelength calibration.
2.4.2 Effective Area
The combination of SiC and LiF coatings on the primary mirrors and
gratings is designed to maximize the effective area across the whole FUV band.
Since the reflectivity of LiF drops rapidly below approximately 1020
Å, the effective area changes significantly with wavelength. In
addition, the gaps between the detector segments creates
narrow bands (typically 10 Å wide) where the total effective area drops
by as much as a factor of 2.
The current effective area is displayed in both
tabular and
graphical formats in the FUSE
Observer's Guide. The FUSE White Paper
The FUSE Flux Calibration describes how our current flux calibration
was derived.
2.4.3 Spectroscopic Resolving Power
Before launch, the resolving power of the instrument was estimated based on measurements made during spectrograph Integration & Test at the University of Colorado. It has been difficult to assess the true spectral resolution on-orbit for several reasons. The smearing effects of the thermally induced image and spectral motions has complicated the analysis, and these motions affect HIST and TTAG data differently. Also, high signal-to-noise data are required, but uncertainties in the detector flat fields, and their effect on measured line widths, need to be assessed. Finally, most real sources do not have intrinsically narrow lines across the bandpass, making a thorough assessment difficult. In addition, the expected resolution is not constant, but rather is a function of both channel and wavelength of interest.
Current estimates of the resolving power can be found in the
FUSE Observer's Guide.
2.4.4 Astigmatism and PSF
An H2 spectrum recorded by flight detector
segment 1B during Spectrograph I&T at the University of Colorado
is shown in Figure 2.4.4-1.
This figure shows the full extent of
the segment in the Y direction (1024 pixels) but only a very small
extent in X (1000 pixels or about 6 Å). This image was "constructed"
by adding together 6 different images, each of which was made with the
lamp source illuminating an individual aperture. From the top to the bottom
of the image, the spectra are LiF (MDRS aperture), LIF (HIRS aperture), LiF (LWRS
aperture), SiC (MDRS aperture), SiC (HIRS aperture), and SiC (LWRS aperture). The pinholes
were not illuminated. The LiF spectra are centered at ~1160Å
while the SiC spectra are centered at ~930 Å.
This image illustrates a number of interesting features of the spectra (some of which are due to the testing setup). First, note that the spectral resolution degrades with the width of the aperture. This is due to the fact that the aperture was fully illuminated during the test, which would more closely match what would be expected in orbit for emission from a large extended source (such as a supernova remnant or planetary nebula). Under conditions of nominal pointing accuracy and jitter, a point source spectrum would have roughly the same spectral resolution in all apertures. Second, notice the astigmatic height and curvature of the spectra in the LiF channel (the upper 3 spectra). This is a natural consequence of the optical design. The narrow aperture is on-axis, and the spectrum through this slit is symmetric about the dispersion axis. Note that the other two LiF spectra are asymmetric since they are off-axis. The spectrum through the MDRS aperture (the top spectrum) is tilted slightly to the left while the spectrum through the LWRS (the 3rd spectrum down from the top of the image) is tilted to the right. The SiC spectra all have much smaller astigmatic heights because the spectra are near a holographic correction point at these wavelengths. The fact that the spectral resolution is lower at a spatial focus point is also obvious from these spectra (compare the widths of the LiF and SiC spectra).
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The spectra discussed above are from side 1 of the instrument. For side 2, the order of the apertures is reversed from what is shown in Figure 2.4.4-1. The LWRS aperture is at the top of the image and the MDRS aperture is below the HIRS aperture. The LiF spectra are still imaged in the upper half of the segment, with the SiC spectra in the lower half.
Two microchannel plate (MCP), double-delay line detectors are used in the FUSE instrument. The MCPs and delay line anodes are curved to match the Rowland circle. In such a detector an incident photon strikes a potassium bromide (KBr) photocathode, which covers the top surface of the high-voltage biased Z-stack of MCPs. The MCPs provide electronic amplification of this initial photoelectron, resulting in a cloud of ~2 × 107 electrons at the bottom of the stack. This electron cloud then impinges on a helical delay line anode below the MCP stack. The events are located in the spectral direction on the anode by measuring the time required for the electron cloud pulse to propagate in the x-direction. Events are located in the spatial direction by measuring how the charge in the cloud is split by the interleaving wedges of the anode in the y-direction.
The total charge that can be extracted from the MCPs is a limited resource that is managed carefully. High S/N observations of any target, bright or faint, deplete this resource and may cause gain sags in the illuminated region of the detector. The FUSE Observer's Guide describes the bright object limitations in greater detail.
The active area of a detector must be approximately 170 × 10 mm in order to capture the full spectral range from a pair of gratings. This requires the use of two MCP stacks ("segments") in each detector. Each segment is 88.5 × 10 mm, and they are placed end-to-end in the long (dispersion) direction, with a ~7 mm gap between the stacks. Due to edge effects, the effective gap size is ~10 mm. The two detectors are displaced slightly in the dispersion direction so that no wavelength interval falls in the gaps of both detectors. Throughout this document the detector segments are labeled 1A and 1B (segments A and B on side 1), 2A and 2B (segments A and B on side 2).
The photon event locations in each detector MCP segment are digitized into 16,384 × 1024 pixels - note that there are no physical pixels on the FUSE detectors; the photon positions are determined by digitizing an analog signal. On all segments, the X pixel size is ~6 µm. However, the Y pixel size is different for the different detector segments. For segments 1A and 1B, the Y pixel size is ~10 µm. For segments 2A and 2B, the Y pixel sizes are larger. The detector resolution, however, is ~25 × 50 µm (i.e. the detector resolution is oversampled by 4 or more).
Information for every photon event is packaged into a 4-byte packet and sent to the Instrument Data System (IDS). Information in this packet includes the detector and segment number of the event, the X,Y location on the segment, and the pulse height of the event.
If the total count rate from the two detectors exceeds 8,000 counts/second in TTAG mode, or 32,000 counts/second in HIST mode, the bus between the detectors and the IDS cannot keep up, and events are randomly discarded at the detector. Spectral integrity is maintained, but photometric accuracy is somewhat compromised. The total number of photons detected (before discarding photons above the bus limit described above) is telemetered to the IDS from the detector. For count rates higher than ~15,000 per segment, the dead-time correction begins to become important (> 20%). In TTAG mode, there is also an upper limit on count rates due to a FIFO. If the count rate is above ~3200 counts per second, the FIFO will eventually fill, and data will be lost. Figure 2.5-1 shows a count rate plot for an exposure where the FIFO fills. The rate is constant until the FIFO fills, then alternates between 0 and 8000 cps.
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Two sets of wire grids lie above the MCPs. A "QE grid", located several millimeters above the surface of the MCPs, creates an electric field above the plates. Photons which strike the MCP between the pores create photoelectrons which do not travel down a pore. The purpose of the QE grid is to deflect these photoelectrons back to the MCP, and thereby prevent a loss in the quantum efficiency of the plates. Deflected photoelectrons return to the grid within ± several pores of where the photon originally hit. Above this QE grid is a "plasma grid", whose job is to keep plasma from the earth's atmosphere from impinging on the MCP. This grid will reject particles with energies up to several volts in energy. Highly energetic particles are not stopped by this grid and result in either normal energy background events in the MCP or high energy events which are rejected by the pulse height discriminators in the detector. The plasma grid gives rise to the "worm" (see below).
A stimulation, or "stim" lamp is located just below the internal spectrograph baffles on each side of the instrument, and about 1 meter above each detector. These lamps are used in orbit as an aid in calibration. The count rate from the lamps are high enough that they can be used to provide some information on the flatfield and distortion properties in the detectors. The shadows of the 2 wire grids are visible in exposures taken with the stim lamps.
The temperature of each detector electronics package is regulated by a connection to a baseplate which is strapped to one of two external radiators. During long observations, the baseplate is typically stable in temperature to < ± 2°, resulting in the electronics being stable to < ± 1°. Large changes in satellite attitude (and solar illumination of the radiators) result in larger absolute changes in the temperature of the baseplate, and therefore affect the temperature of the electronics more strongly. The performance of the electronics is sensitive to changes in temperature, resulting in a "drift" of the measured photons events in detector coordinate space, and a loss of spectral resolution; this drift is corrected for by the CalFUSE pipeline by adjusting the pixel scale based on the position of the two stimulation (or "stim") pulses on each segment, which are electronically inserted into the data stream at the beginning and end of every exposure.
2.5.1 Detector Background
Microchannel plates possess an inherent background rate, which is due
mainly to the beta decay of 40K in the MCP glass. On orbit,
cosmic rays add to this intrinsic background to give a total rate
of ~0.5 counts cm-2 sec-1.
The pulse height distribution (PHD) of background events is different than photon-induced events. Background PHDs have a negative exponential shape, while photon-induced PHDs have a gaussian-like shape. Therefore, it is possible to reject low energy background events with proper settings of the pulse height discriminators. However, within the normal energy range of photon induced events, background events are indistinguishable from photon induced events. The background rate stated above is the value after rejecting the low energy events.
The detector background rate is the ultimate limiting factor in the
ability to detect faint sources.
The contribution of the background to spectra varies with wavelength
in proportion to the astigmatic height of the dispersed spectra.
The FUSE Observer's Guide
describes the current background model and
faint limit, and the FUSE White Paper
Background Subtraction in CalFUSE provides additional details
of the algorithms used.
2.5.2 Detector Flat Field
Flat field images obtained for the FUSE detectors not only measure the
QE response variations as a function of position, but also distortions
and modulations of the true photon event locations at the top of the
MCP. Furthermore, each readout pixel does not really represent an equal
area at the top of the MCP, due to MCP fiber bundle boundary effects,
non-linearities in the anode electronics, and to a small extent the
electron optics behind the MCPs. Some flat Field effects are described
below and in the
FUSE Observer's Guide.
2.6 Fine Error Sensors
Each of the two LiF channels has a Fine Error Sensor (FES) camera that
images the 19.5 × 19.5 arcmin field in the neighborhood of the
science target. Visible light is directed to the FES CCD from the
mirrored front surface of the FPA. The FES determines the centroids of
up to six guide stars in the field with an accuracy of 0.2 arcseconds
and sends this information to the Instrument Data System (IDS) once per
second. This results in
a satellite pointing accuracy of 0.5 arcseconds (rms).
The FES limiting magnitude is V ~ 13.5 (for centroiding to 0.2 arcsec accuracy), sufficient to find at least one guide star in 85% of the fields at the Galactic Poles and virtually 100% of the fields at lower Galactic latitudes. In some fields near the galactic poles it may be necessary to schedule observations at a particular time to maximize availability of guide stars.
Each FES consists of an optical system that re-image the FPA onto a 1024 × 1024 pixel frame transfer SITe CCD, which is masked to a 512 × 512 pixel image area. The pixels are 24 µm square. The CCD is cooled to -60C with a 2-stage thermoelectric cooler coupled to a radiator on the side of the satellite. Only one of the two FESs is used at a time, and the satellite is rolled (around the Z axis) by 2.5° to keep the active radiator in the shade. The other FES will not be used unless dictated by a decline in performance of the primary unit.
Each FES has a three-position filter wheel. In the first position is a nearly zero-loss glass blank, which serves only to maintain the proper optical path length in the system. In the second position is a neutral density filter with an attenuation of 99.5% (6 magnitudes), for use when acquiring bright targets or when guiding on bright stars near the science target. In the third position one FES contains a V-band filter and the other FES contains an R-band filter. There is no shutter on the FES, and consequently the CCD is flushed continuously (when not exposing) to avoid charge build up.
The imaging properties of the FES are modest. The stellar point
spread functions are typically 5 arcsec FWHM, depending on field angle
and color. Nevertheless, an image of the field (with the target either
in or out of the aperture as requested by the investigator) will routinely
be provided to users to confirm that the proper target was acquired.
We expect the photometric calibration of the FES to be good to < 0.1 mag.
2.7 Instrument Data System
The Instrument Data System (IDS) is a redundant, programmable processor
(68020 with floating point coprocessor) that controls the FUSE
instrument. (A "redundant" system contains a primary processor and a
backup processor.) Only one of the two processors is powered on at a
time. The active IDS processor communicates with the spacecraft
subsystems (for example, the spacecraft command and data handling system
and the attitude control system) over a data bus with a maximum data
rate of ~250 kbps. Instrument science and engineering data are sent to
the spacecraft solid-state recorder over this bus. Science data is
allocated up to 120 kbps of the bus traffic. The IDS also receives a 1
Hz signal from the spacecraft that is used to align the IDS clock with
the spacecraft clock to an accuracy of ± 5 millisec.
The IDS communicates simultaneously with all instrument subsystems, and is responsible for controlling all instrument functions, including thermal control, actuators on the mirror assemblies and FPAs, and detector and FES operations. It accepts data from the FUV detectors and FES, and packetizes the data for transmission to the Spacecraft solid-state recorder. The IDS also collects "housekeeping" telemetry (temperatures, voltages, etc) from these subsystems, packetizes them, and sends them to the recorder.
The IDS plays a crucial role in the pointing performance of the
instrument. After a slew to a new target field, the IDS processes the
FES image of this field, and determines the boresight pointing based on
comparison with a star table uplinked from the ground. This measured
pointing is sent to the spacecraft Attitude Control System (ACS) to
update the current pointing. Once the FES begins sending centroid
information to the IDS for a set of guide stars, the IDS computes the
measured quaternion (pointing vector) once every second and sends it to
the ACS to maintain pointing stability.
2.8. The Spacecraft
The FUSE spacecraft bus is a modified version of the Explorer platform,
which was used to support the Extreme Ultraviolet Explorer
(EUVE) mission. The FUSE spacecraft also includes heritage from the
X-ray Timing Explorer (XTE) and Tropical Rainfall Measurement
Mission (TRMM) missions in the attitude control system and
communications. The spacecraft was built by Orbital Sciences
Corporation.
2.8.1 Mechanical System
The spacecraft bus is approximately 0.9
meters tall and 1.8 meters in diameter, with a mass of 580 kg. The
mechanical subsystem consists of the primary spacecraft structure, the
solar arrays, and mechanisms. The structure is an aluminum trapezoidal
frame that interfaces with the instrument through three flexure mounts.
The only mechanisms in the spacecraft are the solar array hinge and its
actuator release system and the solar array drives. Each
solar array panel uses an array drive to orient the panel to the sun.
The drives are actuated only during major slews so that the
pointing stability is not disturbed during observations.
2.8.2 Command & Data Handling (C&DH) System
The spacecraft C&DH system provides the
hardware and software necessary to:
The C&DH processor is a 80386 microprocessor with 1 MByte of
radiation-hardened, single-event-upset immune, random access memory for
program execution. The bulk memory card provides 240 MBytes of
data storage (of which slightly over half is reserved for the storage of
scientific data).
2.8.3 Attitude Control System (ACS)
The ACS subsystem consists of two 3-axis Ring Laser Gyro inertial
reference units, two 3-axis magnetometers, two coarse sun-sensor units,
three magnetic torquer bars, and four reaction wheel assemblies. FUSE
does not employ any star-trackers. Safehold software is resident in the
ACS; if the ACS detects that the pointing of the satellite violates the
beta angle restrictions, then the spacecraft will autonomously be
commanded into a safehold mode. The beta angle is the angle measured
from the anti-solar direction to the boresight and is nominally
restricted to values of 15-105°. See the discussion of observing
constraints in the FUSE Observer's Guide.
The ACS provides autonomous control of the satellite and maintains
pointing control to < 2° in coarse pointing mode (without
information from the instrument Fine Error Sensor (FES)), and controls
pointing jitter to < 0.5 arcsec (1 sigma) with a reference position
measurement provided by the FES. The ACS enables the satellite to slew
at a rate of up to 4° per minute, so even 90° slews can be
completed during an occultation period. After a large slew (i.e., when
going from one target to another), the uncertainty in boresight pointing
will typically be 0.02*TSLEW arcmin, where TSLEW is the slew time in
minutes. Short slews, which are typically performed during the target
acquisition peakup sequence, are expected to be accurate to ~0.1
arcsecond. The predicted (corrected) gyro drift rate of the satellite
is approximately 30 milliarcsec/sec. The IDS, however, provides
reference position measurements to the ACS (measured quaternions from
observations of guide stars by the FES) and allows the ACS to correct
the pointing and keep the target in the chosen spectrograph aperture with a
jitter of < 0.5 arcsec (rms) over one orbit.
2.8.4 Power Subsystem
The power system for FUSE consists of two 40 Amp-hour batteries, two
solar array panels, and associated electronics. The batteries employ a
standard 22-cell NiCd arrangement. The batteries provide enough power
for FUSE to operate normally during the 35 minute eclipse period every
orbit. The solar arrays use Gallium Arsenide (GaAs) cells to provide
520 Watts of orbital-average-load power.
2.8.5 RF Communications Subsystem
The communications system consists of two omni-directional low-gain antennas
and two S-band transponders with associated diplexers. The system
interfaces with the central electronics unit of the C&DH. The
system receives uplinked commands at 2 kbps and can downlink telemetry
at a variety of data rates (up to 1 Mbps). The two omni-antennas are
located on opposite sides of the spacecraft bus and provide almost
spherical coverage.
In photon address mode (also known as time-tag or TTAG mode), the X and Y locations, arrival time, and pulse height of each detected event are stored as a photon list. The data are acquired as follows: After the target is centered in the aperture, the Instrument Data System (IDS) begins storing the photon events recorded by the detector. Each photon event word is 32 bits long, and contains the x location (14 bits), y location (10 bits), pulse height (5 bits), and detector and segment ID (2 bits). The range of x and y locations of recorded photon events can be limited by the detector ``masks.'' There are 4 such masks:
Each segment has its own set of 4 detector masks. In TTAG mode, science data are obtained with the ``active image'' mask, which normally covers the entire active area of the detector. Therefore, photon events from all the apertures are recorded. One aperture in each channel contains the spectrum of the target, while the other apertures only contain spectra of the sky plus airglow.
The photon event words are stored in the IDS memory, then packaged for transmission to the spacecraft recorder. The IDS also inserts time markers in the data stream at a regular interval (the frequency is determined by ground command, with a default of one second). The IDS inserts header information in the science packets, which allow the ground to identify the data (i.e., the program ID, the observation number and the exposure number). Finally, the IDS inserts ``engineering snapshots'' into the science data stream at periodic intervals (again determined by ground command). It should be noted that these snapshots contain only instrument housekeeping data - the IDS does not have access to spacecraft housekeeping data. Furthermore, these snapshots are inserted at a low frequency (nominally once every 300 seconds), and are not adequate to remove high frequency effects such as pointing jitter. The information is useful for monitoring temperature changes in the detector during the exposure, for instance.
After the data are transmitted to the ground, they make their way to the Science Data Processing pipeline, where the science packets are cracked open, and the photon event data are sorted into different files depending on the detector and segment IDs. The pipeline performs bookkeeping and takes reasonable automated steps to deal with missing or corrupted data. The engineering snapshot packets are also written to separate associated files, for later use by the calibration software.
Photon address data are stored in FITS binary tables, with one detector segment per FITS file. If all the segments are in operation, there are four FITS files per exposure. These files can be quite large - several million photon events per exposure for a bright target.
Observing bright targets in TTAG mode can rapidly use up the available space on the spacecraft recorder. Consequently, when the predicted UV flux of a target is large enough that it would produce more than 2500 counts/s from all detector segments combined, the IDS is commanded to store the data in spectral image mode (also referred to as histogram or HIST mode). Photons are accumulated in bins, which can be larger than one detector pixel, and hence the arrival time of each photon is lost.
With a bin size of 1 x 1 (i.e. one detector pixel), a spectral image for a single detector segment would be 16384 x 1024 pixels (32 MB) in size, or 128 MB for all four segments! Because this is so large, it is not possible to store the data for all segments at full resolution. Instead, a Spectral Image Allocation (SIA) table, describing the region of each detector segment to save and what binning size to use, is uploaded to the IDS before each HIST observation. This SIA table is essentially a mask made up of 512 rectangles. The size of each rectangle is 2048 detector readout pixels in length (x) and 16 detector readout pixels in height (y). The SIA table specifies which of these rectangles should be activated (mask bit set ``on'') - i.e. if the IDS receives a photon event from the detector whose (x,y) coordinates map to a location in an active rectangle, then that photon event is stored in bulk memory by incrementing a counter in a histogram bin at the resolution specified by ground command. By default, a histogram binning size of 1 x 8 is used. The default SIA table specifies storage of a region around the aperture containing the target, and requires ~20 MB of storage for an orbit's worth of exposures (typically four, see below). Note that in spectral image mode, only data taken through the active science apertures are recorded.
The spectral images are stored as two-dimensional FITS extensions. For each raw spectral image reconstructed by the Science Data Processing pipeline, the necessary information about the image's location on the detector is written into the FITS header. The binning factor is also recorded in the header.
Since Doppler compensation is not performed on-board, spectral image exposures must be kept short to avoid losing spectral resolution. When observing in the direction of satellite motion, there is a 15 km/s difference from one side of the orbit to the other. So, to avoid smearing of the spectra, an orbital viewing interval is divided into four exposures.
All FUSE science data products are files in FITS format with the data stored as extensions. Seven basic types of files emerge from the Science Data Processing pipeline:
The data can be retrieved from the FUSE data archive, which is hosted by the Multi-Mission Archive at the Space Telescope Science Institute (MAST). In addition to the standard products listed above, the archive contains calibration data files, programmatic data such as proposal titles, abstracts, investigators, etc., and a trailer file giving a log of data processing operations performed.
All files produced and used by OPUS and by the Calibration Pipeline have a unique root name based on the naming convention used in the Mission Planning system for managing program IDs, targets and exposure numbers. The first eleven characters of all file names are the same format, which we call the root name: ppppttooeee, where pppp is the proposal ID (also called the program ID), tt is the target ID, oo is the observation ID, and eee is the exposure ID (all assigned by Mission Planning). Imagine that proposal A102 has three targets which are to be visited four times each with five exposures per visit. The root name of the fifth exposure (eee=005) of the fourth observation (oo=04) of the third target (tt= 03) in proposal A102 (pppp=A102) would be A1020304005.
Table 4.1-1 includes the names of all files that are produced and archived by the Science Data Processing system for a single TTAG exposure with the root name of our example, A1020201004. The large number of files produced is due to the extraction of spectra from all apertures, although the target is located in only one aperture. The spectra taken through the other apertures provides a measure of airglow contamination. In some cases, the other apertures may also contain astrophysically interesting data; for instance, if the telescope was pointed at an extended source, such as a supernova remnant.
A typical observation consists of multiple exposures. In addition to the calibrated, extracted spectra from each detector segment, co-added spectra are made available. These take the form of co-added spectra from the different spectrograph channels in a single exposure, as well as co-added spectra from all exposures (called an observation summary spectrum).
Table 4.1-2 provides filenames for HIST exposures. There are many fewer files produced here because data from the apertures not containing the target are not saved on-board. In the following sections, the file naming syntax is described.
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Table 4.1-1. Address stream files for root exposure A1020201004
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Table 4.1-2. Spectral Image files for HIRS root exposure A1020201004
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Fine error sensor (FES) images have filenames with syntax ppppttooeeeaaaafxxx.fit, where aaaa is always either fesa or fesb, depending on whether the image was obtained with camera FES-A or FES-B. The section xxx is a processing step flag, and is either raw, for final raw data, or cal, for final calibrated data. Using our example root name above, a calibrated FES-A camera image would have filename A1020201004fesafcal.fit.
Raw files of both address stream (time-tagged) and spectral image data will have filenames of the form ppppttooeeessaaaafraw.fit. ss indicates the detector segment and has allowed values of 1a, 1b, 2a, 2b or 00 (00 indicates that this is a final merged spectrum). aaaa is a 4-character designator with values of ttag for address stream data or hist for spectral images. Continuing with the example root name above, an address stream data file from segment 2B would have filename A10202010042bttagfraw.fit.
Extracted spectra will have yet another field added to their roots to indicate which channel and aperture they came from: ppppttooeeessccccaaaafxxx.fit. cccc can have values of sic1, sic2, sic3, sic4, lif1, lif2, lif3, lif4 or all[1-4]. The all[1-4] designation will be used for the merged spectrum, and in this case ss will also equal 00 as noted above. The numbers refer to the extracted aperture as follows:
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Table 4.1-3. Extracted aperture numbering convention
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An example filename for a SiC 2B spectrum taken through the HIRS aperture
would be
A10202010042bsic3ttagfcal.fit.
A merged ttag spectrum of a target in the HIRS aperture for a single exposure (i.e. all channels co-added) would have a filename A102020100400all3ttagfcal.fit. Similarly, the merged ``observation level'' spectrum (all channels co-added for all exposures in that observation) would have filename A102020100000all3ttagfcal.fit
Engineering snapshot data filenames have syntax ppppttooeeeaaaaf.fit, where aaaa is a 4-character designator with values of snap, for standard engineering snapshot files, snpa for engineering snapshots associated with FES-A, and snpb for engineering snapshots associated with FES-B. An example filename would be A1020201004snapf.fit.
Engineering time-resolved data filenames have syntax ppppttooeeeaaaaf.fit, where aaaa is a 4-character designator with values of hskp for the engineering housekeeping telemetry data files, and jitr for jitter data files. An example filename would be A1020201004hskpf.fit or A1020201004jitrf.fit.
The FUSE data is archived at the Multimission Archive at the Space Telescope Science Institute (MAST). The data can be accessed from the MAST web page at:
The pages and forms are self explanatory.You can search for a dataset in many ways - by program ID, target name, coordinates, etc. If you choose to search by target name, be wary of the object name Resolver. For instance, if you enter the star name SK-67D169, and ask Simbad to resolve the name, it fails to find anything. It turns out that Simbad will only work on this particular name if you enter SK-67169. If you know the exact name of the target as it was entered on the Phase 2 form (and the way it appears in the FITS header), then enter the target name and do not use the Resolver at all.
If you decide to search for the dataset using the program ID, then you can search using Program or Observation Name. If, for instance, you want to see all observations from program P103, then just enter P103 in the form under Program, and hit the search key. If you want to find a specific observation in P103, say P1031603, then in the Observation Name field enter P1031603000 (note the 3 trailing 0's appended to the string). Entering P1031603* also works.
All FUSE data files are FITS files, and use extensions. With one exception, all files have empty primary arrays, and the data starts in the first extension. All files have some basic information in the primary FITS header, however, and all files contain further information in the extension headers.
We describe here the standard keywords in the primary FITS header of raw and calibrated exposures. The FITS keywords are organized into sections in the header. The first such set of keywords are the DATA DESCRIPTION KEYWORDS shown below. Note that the keywords in these files are updated from time to time, so there may be slight differences between this keywords in this list and those in your data files.
DATA DESCRIPTION KEYWORDS
TELESCOP= 'FUSE ' / Telescope used to acquire data
INSTRUME= 'FUV ' / Instrument in use (one of FUV, FESA, or FESB)
ROOTNAME= 'Z9042701001 ' / Rootname of the observation set (ppppttooeee)
FILENAME= 'Z90427010011attagfraw.fit' / Filename
FILETYPE= 'RAW PHOTON ADDRESS' / Type of data found in data file
EXP_STAT= 0 / Science Data Processing status (0=good)
These keywords are fairly self-explanatory. The PROPOSAL INFORMATION keywords come from the Mission Planning Database (MPDB) and are shown below:
PROPOSAL INFORMATION
PRGRM_ID= 'Z904 ' / Program ID from data header
TARG_ID = '27 ' / Target ID from data header
SCOBS_ID= '01 ' / Observation ID from data header
EXP_ID = '001 ' / Exposure ID from data header
OBS_ID = '01 ' / Observation ID from proposal database
PR_INV_L= 'Dupuis ' / Last name of principal investigator
PR_INV_F= 'Jean ' / First name of principal investigator
The TARGET INFORMATION keywords are populated from the MPDB and are not measured quantities. The expected fluxes and count rates come from the Phase 2 form submitted by the observer.
TARGET INFORMATION
TARGNAME= 'PG1610+519 ' / Target name on proposal
RA_TARG = 243.001667 / [deg] Right ascension of the target (J2000)
DEC_TARG= 51.829028 / [deg] Declination of the target (J2000)
APER_PA = 221.871582 / [deg] Position angle of Y axis (E of N)
EQUINOX = 2000.0 / Equinox of celestial coord. system
ELAT = 7.010827768080E+01 / [deg] Ecliptic latitude
ELONG = 2.144550863054E+02 / [deg] Ecliptic longitude
GLAT = 4.531478538086E+01 / [deg] Galactic latitude
GLONG = 8.050825777753E+01 / [deg] Galactic longitude
OBJCLASS= 28 / Object class (modified IUE classification)
SP_TYPE = 'sdB ' / MK spectral type and luminosity class (or other
SRC_TYPE= 'PC ' / [P]oint or [E]xtended, [C]ontinuum, [E]mission
VMAG = 13.74 / Target V magnitude
EBV = 0.05 / Color excess E(B-V)
Z = 0.0 / Red shift
EFLX_950= 1.19E-12 / Expected flux at 950 A
EFLX1050= 3.32E-12 / Expected flux at 1050 A
EFLX1150= 2.54E-12 / Expected flux at 1150 A
HIGH_PM = 'F ' / High proper motion target (T or F)
MOV_TARG= 'F ' / [F]ixed or [M]oving target
PCNTRATE= 988.0 / Expected count rate (sum of all channels)
The SUMMARY EXPOSURE INFORMATION contains the date and time of the observation, including the exposure start and end time, and the computed exposure time. Exposure start/end times are expressed in Modified Julian Date (MJD = JD-2400000.5). The EXPENDC keyword indicates the reason for the termination of the exposure - entry into South Atlantic Anomaly (SAA), occultation (OCC), end of night (NGHT), or the natural end of the observation (EOBS). The sun and moon angles, and the minimum pointing angle above the earth limb are computed by the calibration software, and are therefore 0 in the raw data files. The calibration software also screens photon address data, and potentially deletes `bad'' data taken in, for instance, the SAA. There are a number of keywords reporting the actions taken by this screening step, and are shown below.
SUMMARY EXPOSURE INFORMATION
DATEOBS = '2002-07-16' / UT date of start of exposure (yyyy-mm-dd)
TIMEOBS = '19:25:28' / UT start time of exposure (hh:mm:ss)
EXPSTART= 5.247180935185E+04 / Exposure start time (Modified Julian Date)
EXPEND = 5.247183303856E+04 / Exposure end time (Modified Julian Date)
EXPTIME = 1.234461E+03 / [sec] Exposure duration -- calculated
EXPENDC = 'ENDE ' / End condition (ENDO; SAA; OCC; NGHT; ENDE)
NEVENTS = 80096 / Number of events in raw data file
RAWTIME = 0.000000 / [sec] Exposure duration of raw data file
PLANTIME= 2079 / [sec] Planned exposure time
SUNANGLE= 9.35174335378395E+01 / [deg] Angle between sun and Z axis
MOONANGL= 6.73868835568468E+01 / [deg] Angle between moon and Z axis
GEO_LONG= -1.28664211133360E+02 / [deg] Geocentric longitude
GEO_LAT = 2.08888960657134E+01 / [deg] Geocentric latitude
MAG_LAT = 1.47165648743697E+01 / [deg] Geomagnetic latitude
MIN_LIMB= 1.32762149512837E+01 / [deg] Minimum angle above Earth limb
NBADEVNT= 28105 / Number of events deleted in screening (TTAG onl
NBADPHA = 0 / Number of events deleted in PHA screening (TTAG
EXP_BAD = 8.100000E+02 / [s] Integration time lost to screening
EXP_SAA = 0.000000E+00 / [s] Integration time while in SAA
EXP_LIM = 8.100000E+02 / [s] Integration time with low limb angle
EXP_BRST= 0.0 / [s] Integration time lost to event bursts
EXP_JITT= 0.0 / [s] Integration time lost to jitter
EXPNIGHT= 0.000000E+00 / [s] Integration time during night after screeni
V_GEOCEN= 1.14309341647768E+00 / [km/s] Observed to geocentric velocity
V_HELIO = 9.81311996043170E+00 / [km/s] Geocentric to heliocentric velocity
V_LSRDYN= -1.43436927795410E+01 / [km/s] Heliocentric to dynamical LSR
V_LSRSTD= -1.73173561096191E+01 / [km/s] Heliocentric to standard solar LSR
The SCIENCE INSTRUMENT CONFIGURATION keywords contain planned as well as measured (ie. telemetered) values for instrument configuration. Most of the keywords shown below are self-explanatory. Detector mask AIC1 is ``open'' - none of the photon events are masked out by the detector. The focal plane assembly (FPA) X and Z positions from the telemetry are recorded in the header keywords.
SCIENCE INSTRUMENT CONFIGURATION
INSTMODE= 'TTAG ' / Instrument mode (TTAG or HIST)
DETECTOR= '1A ' / Detector for data in this file (1A, 1B, 2A, 2B)
APERTURE= 'LWRS ' / Planned target aperture (PINH, HIRS, MDRS, LWRS
APER_ACT= 'LWRS_LIF' / Extracted aperture in this file
DET_MASK= 'AIC1 ' / Detector mask used
NUM_APS = 1 / Number of apertures in use in the observation
INITBINX= 1 / Initial binning in X coordinate
INITBINY= 1 / Initial binning in Y coordinate
SPECBINX= 1 / Current binning in X coordinate
SPECBINY= 1 / Current binning in Y coordinate
SIA_TBL = ' ' / SIA Table name
NUM_HIST= 0 / Number of histograms in the observation
INT_CAL = ' ' / Internal Calibration (STIM, NONE)
FPSPLIT = 0 / Number of FP Split positions
FPASXPOS= 260.153 / [microns] FPA SiC X position
FPALXPOS= 117.243 / [microns] FPA LiF X position
FPASZPOS= -99.6363 / [microns] FPA SiC Z position
FPALZPOS= -34.7361 / [microns] FPA LiF Z position
TTPERIOD= 0.008 / [s] Time marker insertion period
ENGSNAPT= 300 / [s] Engineering snapshot period
TACQSEQ = 'ACQ-T-NOPKUP ' / Target acquisition sequence
ROLLOFF = 0.0 / [Deg] Roll offset
FES_BIN = 1 / FES binning factor (1;2;4;8)
FES_FILT= 'CLR ' / FES filter wheel position: CLR=Clear, COL=Color
FES_TPOS= 'IN ' / FES target position: IN or OUT
The Detector HV bias ranges keywords list the minimum and maximum high voltage level for all segments during the exposure. The minimum and maximum for a given segment should normally be the same. The high voltage is adjusted periodically. See the FUSE White Paper Time-Dependent Calibration Effects for the high voltage values as a function of time.
Detector HV bias ranges
DET1HVAL= 154 / Det1 MCP A HV bias minimum setting
DET1HVAH= 154 / Det1 MCP A HV bias maximum setting
DET1HVBL= 149 / Det1 MCP B HV bias minimum setting
DET1HVBH= 150 / Det1 MCP B HV bias maximum setting
DET2HVAL= 148 / Det2 MCP A HV bias minimum setting
DET2HVAH= 148 / Det2 MCP A HV bias maximum setting
DET2HVBL= 118 / Det2 MCP B HV bias minimum setting
DET2HVBH= 118 / Det2 MCP B HV bias maximum setting
HV_FLAG = 0 / 4 = full voltage for entire exposure
The ORBITAL EPHEMERIS keywords contain the Keplerian elements for the FUSE orbital ephemeris. Together with the time of observation, these values can be used to calculate where the satellite was in its orbit. The calibration software, for instance, uses these values to compute the Doppler correction which has been applied to the calibrated data.
ORBITAL EPHEMERIS
EPCHTIMD= 5.24710000000000E+04 / Epoch time of parameters whole day (MJD)
EPCHTIMF= 2.43629490003514E-01 / Epoch time of parameters fraction of day (MJD)
INCLINAT= 2.49831000000000E+01 / [deg] Inclination
ECCENTRY= 1.05500000000000E-03 / Eccentricity
MEANANOM= 3.56099800000000E+02 / [deg] Mean anomaly
ARGPERIG= 3.96000000000000E+00 / [deg] Argument of perigee
RASCASCN= 3.40016600000000E+02 / [deg] RA of ascending node
SEMIMAJR= 7.13345011877659E+03 / [km] Semi-major axis
MEANMOTN= 1.44096472900000E+01 / [revs/day] Mean motion
FDM2COEF= 4.44000000000000E-06 / First time derivative of mean motion / 2
SDM6COEF= 0.00000000000000E+00 / Second time derivative of mean motion / 6
DRAGCOEF= 8.09880000000000E-05 / B star drag coefficient
PROPMODL= 'SGP4 ' / Propagation model
The SCIENCE DATA PROCESSING STEPS keywords indicate which steps of the calibration pipeline have been applied to the data. All processing steps are set to a value of INITIAL in the raw data files. The first pipeline module sets them to either PERFORM or OMIT, depending on whether the corresponding calibration step is to be performed or not. Once a step is completed, its keyword is changed to a value of COMPLETE. If a pipeline module decides not to perform a step labeled PERFORM, it changes the keyword to SKIPPED. Thus, COMPLETE, OMIT, and SKIPPED are all valid keywords in extracted spectral files.
The SCIENCE DATA PROCESSING CALIBRATION FILES keywords indicate the names of the SDP calibration files used by the calibration software. Knowing these filenames is important if one is considering reprocessing the data.
The FUV SCIENCE DATA PROCESSING KEYWORDS section collects other data which is relevant to the calibration.
FUV SCIENCE DATA PROCESSING KEYWORDS
OPUSVERS= '2.5 ' / OPUS version number
CF_VERS = '1.8.7 ' / CALFUSE pipeline version number
HKEXISTS= 'yes ' / Housekeeping data file exists
JACDRIFT= 9.99831108965706E-01 / Jacobian of drift correction
PHALOW = 0 / Pulse height screening low limit
PHAHIGH = 31 / Pulse height screening high limit
OPT_EXTR= 3 / Number of optimal extraction iterations
STIM_L_X= 2.303575E+02 / X centroid of left stim pulse
STIM_L_Y= 8.062679E+02 / Y centroid of left stim pulse
STIM_R_X= 1.611667E+04 / X centroid of right stim pulse
STIM_R_Y= 7.823282E+02 / Y centroid of right stim pulse
SPECYCNT= 0.0 / Y centroid of target spectrum
WORM_BEG= 0 / [pixels] Start of worm region
WORM_END= 0 / [pixels] End of worm region
WPC = 0.0 / [A] Wavelength increment per pixel
W0 = 0.0 / [A] Wavelength of first pixel
TOTAL_CR= 3.9137444E+01 / NEVENTS/EXPTIME for raw data file
DET_DEAD= 1.001112E+00 / Detector deadtime correction factor
IDS_DEAD= 1.000000E+00 / IDS deadtime correction factor
TOT_DEAD= 1.001112E+00 / Total deadtime correction factor
A fair amount of the detector housekeeping (engineering telemetry) data is included in the FITS headers. The values included are shown below. Except for the active image counters, only those values from the relevant detector segment are inserted into the header. The example keywords below are for segment 1A.
ENGINEERING KEYWORDS
THERMCON= 'ON ' / Thermal control flag
FESSRC = 'FES A ' / Source of FES dump data
FESCENT = 'FES A ' / Source of FES centroid data
GMODE = 'IDLE ' / Guidance mode
TRACKON = 'OFF ' / Guide star tracking on
ATCMDATT= 'AT COMMANDED ATTITUD ' / At commanded attitude
Segment 1A Counters
C1ASIC_B= 12837609 / SIC counter at beginning of integration
C1ASIC_E= 12850970 / SIC counter at end of integration
C1ALIF_B= 15942236 / LIF counter at beginning of integration
C1ALIF_E= 15971907 / LIF counter at end of integration
C1ADE_B = 2968196 / Digitized events counter at beginning of integr
C1ADE_E = 3048310 / Digitized events counter at end of integration
C1AAS_B = 4685204 / Autonomous shutdown (SAA) counter at beginning
C1AAS_E = 4687983 / Autonomous shutdown (SAA) counter at end of int
C1AFE_B = 7010558 / Fast event counter at beginning of integration
C1AFE_E = 7100932 / Fast event counter at end of integration
C1AFE_CR= -1 / Count rate from fast event counter
Active Image Counters
CTIME_B = 5.247180935185E+04 / [MJD] Time of first engineering snapshot
CTIME_E = 5.247183303241E+04 / [MJD] Time of last engineering snapshot
C1AAI_B = 2964857 / Seg 1A at beginning
C1AAI_E = 3044971 / Seg 1A at end of integration
C1BAI_B = 4586341 / Seg 1B at beginning
C1BAI_E = 4635234 / Seg 1B at end of integration
C2AAI_B = 13354443 / Seg 2A at beginning
C2AAI_E = 13397415 / Seg 2A at end of integration
C2BAI_B = 11544600 / Seg 2B at beginning
C2BAI_E = 11597186 / Seg 2B at end of integration
Detector 1 parameters
DET1ETMP= 25.6041 / DET1 electronics temperature
DET1PTMP= 18.5962 / DET1 Baseplate temperature
DET1ASCL= 17.0 / DET1 segment A Time (X) image scale factor
DET1BSCL= 92.0 / DET1 segment B Time (X) image scale factor
DET1AXOF= 183.0 / DET1 segment A Time (X) image position offset
DET1BXOF= 107.0 / DET1 segment B Time (X) image position offset
DET1TEMP= 19.2937 / DET1 Detector temperature
DET1HVMT= 24.91 / DET1 High Voltage Module temperature
DET1HVFT= 21.386 / DET1 High Voltage Filter temperature
DET1AMAT= 26.7594 / DET1 segment A Amplifier temperature
DET1AMBT= 26.7594 / DET1 segment B Amplifier temperature
DET1TDAT= 21.792000 / DET1 TDC A temperature
DET1TDBT= 21.792 / DET1 TDC B temperature
DET1HVA = 65.0 / DET1 MCP A Bias Current
DET1HVB = 53.0 / DET1 MCP B Bias Current
DET1AUCT= 227.0 / DET1 TDC-A Upper Charge Threshold setting
DET1BUCT= 228.0 / DET1 TDC-B Upper Charge Threshold setting
DET1ABWK= 165.0 / DET1 TDC-A Begin CFD Walk setting
DET1BBWK= 139.0 / DET1 TDC-B Begin CFD Walk setting
DET1AEWK= 165.0 / DET1 TDC-A End CFD Walk setting
DET1BEWK= 125.0 / DET1 TDC-B End CFD Walk setting
DET1ABSL= 227.0 / DET1 TDC-A charge Baseline threshold setting
DET1BBSL= 228.0 / DET1 TDC-B charge Baseline threshold setting
DET1ALCT= 7.0 / DET1 TDC-A Lower Charge Threshold setting
DET1BLCT= 7.0 / DET1 TDC-B Lower Charge Threshold setting
DET1ALTT= 169.0 / DET1 TDC-A Lower Time Threshold setting
DET1BLTT= 143.0 / DET1 TDC-B Lower Time Threshold setting
DET1HVGR= 1.0 / DET1 Grid High Voltage Status
DET1HVPW= 1.0 / DET1 Bias High Voltage Status
DET1STIM= 'OFF ' / DET1 Stimulation Lamp Status
Finally, keywords used by the archive, and those necessary to associate exposures are included.
ARCHIVE SEARCH KEYWORDS
BANDWID = 9.655731E+01 / [Angstroms] Bandwidth of the data
CENTRWV = 1.035004E+03 / [Angstroms] Central wavelength of the data
WAVEMIN = 9.867254E+02 / [Angstroms] Minimum wavelength of the data
WAVEMAX = 1.083283E+03 / [Angstroms] Maximum wavelength of the data
PLATESC = 7.680400E-01 / [arcsec/pixel] Plate scale
ASSOCIATION KEYWORDS
ASN_ID = 'Z9042701000 ' / Unique identifier assigned to association
ASN_TAB = 'Z9042701000asn.fit ' / Name of the association table
ASN_MTYP= 'EXPOSURE ' / Role of the exposure in the association
Raw address stream data are stored as a FITS binary table. As is customary for this data format, the primary array is empty and the table is stored in the first extension. The first group of FITS keywords from extension 1 are shown below.
XTENSION= 'BINTABLE' / Binary image extension BITPIX = 8 / Bits per pixel NAXIS = 2 / Number of data axes NAXIS1 = 9 / Size of the first axis NAXIS2 = 292617 / Size of the second axis PCOUNT = 0 / Size of special data area GCOUNT = 1 / One data group (required keyword) TFIELDS = 4 / Number of fields in each row EXTNAME = 'EVENTS ' / Name of this binary table extension EXTVER = 1 / Extension version number BUNIT = 'UNITLESS ' / Brightness units RA_TARG = 243.001667 / Right ascension of the target (deg) (J2000) DEC_TARG= 51.829028 / Declination of the target (deg) (J2000) PA_APER = 0.000000000000E+00 / Position Angle of reference aperture center EQUINOX = 2000.0 / Equinox of celestial coord. system
BITPIX is always set to 8 for binary tables. NAXIS2 indicates the number of photons in this time-tagged exposure (292617 photons in the above example). There is one row in the table for each recorded photon, and each row has TFIELDS = 4 fields of data. These fields include the TIME, X, Y, PHA values for each photon. PHA is the pulse height (energy) of the electron cloud. The data formats and units are given in FITS keywords in this extension. The X coordinate is telemetered as a 14 bit value but is stored as 16 bits in the FITS file. Similarly, Y contains only 10 bits of information, and PHA is really a 5 bit number. These values are all stored on word boundaries in the FITS files.
TIMETAG EVENTS TABLE COLUMNS
TTYPE1 = 'TIME ' / Event clock time
TFORM1 = '1E ' / Data format for TIME: 32-bit float
TUNIT1 = 'seconds ' / Units for TIME: seconds
TTYPE2 = 'X ' / X coordinate
TFORM2 = '1I ' / Data type for X: 16-bit integer
TUNIT2 = 'pixels ' / Physical units for X: pixels
TTYPE3 = 'Y ' / Y coordinate
TFORM3 = '1I ' / Data format for Y: 16-bit integer
TUNIT3 = 'pixels ' / Physical units for Y: pixels
TTYPE4 = 'PHA ' / Measured photon pulse height
TFORM4 = '1B ' / Data format for pulse height: 8 bits
TUNIT4 = 'UNITLESS' / Units for pulse height
Finally, the extension header also contains an exposure time summary giving the total ``live'' time. For raw data, the total exposure time XS-ONTI is always equal to the live time XS-LIVTI. The calibration software can modify the data and change these values.
EXPOSURE TIME SUMMARY
XS-MJDRD= 52471 / MET start (MJD) (day)
XS-MJDRF= 7.9075229E-01 / MET start (MJD) (day fraction)
XS-ONTI = 1.423531250000E+03 / Total exposure time (s)
XS-LIVTI= 1.423531250000E+03 / Live exposure time (s)
XS-DTCOR= 1.0 / Dead time correction factor
The raw spectral image files have a different structure than the address stream binary tables, aside from the fact that the data are images. Each spectral image file contains a primary array containing an image of the SIA table, plus a number of image extensions. The exact number of extensions is determined by the SIA table itself. Each set of contiguous rectangles (each ``rectangle'' has size 2048 x 16 detector pixels; see the discussion above) constitutes an image which occupies its own FITS extension. In cases where the entire detector segment is imaged, the raw spectral image file will have 3 FITS extensions:
Flat field images typically have this kind of data structure. SIA tables used for normal spectral observations store data in two distinct regions of the detector - a set of contiguous rectangles around the SiC aperture containing the target, and a corresponding set around the LiF aperture. So, a standard spectral image of an astronomical source will have 4 image extensions:
The SIA table image stored in the primary FITS array is really a ``bit mask'' showing which parts of the detector are saved. Pixel (rectangle) values of 1 indicate that photons within that rectangle are saved, while values of 0 indicate that they are not stored. The size of the SIA table image is 8 x 64 pixels (rectangles).
The first FITS extension contains the spectral image in the SiC channel for standard SIA tables. As a default, spectral images are binned by a factor of 8 in Y and unbinned in X (the dispersion direction). Also, as a default in detector 1, the spectral image size in Y is 1600 microns or 160 unbinned pixels in Y (the size is different for other channels). With a binning of 8, this yields an image size of 16384 x 20 binned pixels. On detector 2, the detector pixel size in Y is larger, and hence the spectral image size in binned pixels is smaller; the default spectral image size is 16384 x 14 binned pixels.
Example image extension keywords for extension 1 of detector 1A are shown below:
XTENSION= 'IMAGE ' / binary table extension
BITPIX = 16 / bits per data value
NAXIS = 2 / number of data axes
NAXIS1 = 16384 / size of the first axis
NAXIS2 = 20 / size of the second axis
PCOUNT = 0 / size of special data area
GCOUNT = 1 / one data group (required keyword)
EXTNAME = 'HISTOGRAM ' / name of this binary table extension
EXTVER = 1 / extension version number
BUNIT = 'COUNTS ' / brightness units
BZERO = 32768 / image brightness offset
BSCALE = 1 / image brightness scale
HISTOGRAM REGION PARAMETERS
XORIGIN = 0 / offset of this region in the x dimension
YORIGIN = 24 / offset of this region in the y dimension
RCOUNT = 80 / count of rectangles in this region
MINVAL = 0 / minimum value within rectangles
MAXVAL = 435 / maximum value within rectangles
The data are stored as unsigned integers (hence BZERO = 32768.) The values of XORIGIN, YORIGIN give the coordinates of the lower left corner of the spectral image, so that its location within the detector segment can be determined. For default SIA tables, XORIGIN is always 0, meaning that the spectral image starts at the left edge of the detector segment. In the example above YORIGIN is 24 meaning that the lower edge of the spectral image is 24 rectangles in height from the bottom of the detector segment. In detector pixels, this is at Y = 24*16 = 384.
The image of the LiF spectrum is stored in FITS extension 2. It has similar keyword values as above, except of course for the value of YORIGIN, which is larger (the LiF spectrum is imaged along the top half of the detector segment).
Finally, FITS extensions 3 and 4 contain ``cut-out'' images of the detector stim pulses. These are one rectangle in size, so with a binning of 1 x 8, they have an image size of 2048 x 2 binned pixels. Since the stims are located on the upper half of the detector segment, the rectangles containing the stim pulses will overlap the ends of the LiF spectra.
Extracted, calibrated spectra are stored in the first extension of the FITS file as a binary table with six columns giving wavelength (in Å), flux (erg cm^{-2} s^{-1} Å), one sigma errors on the flux, quality flags, total counts, and one sigma errors on the counts. Each aperture on each detector segment will be extracted to a separate file, with a filename as described previously.
An FES image will be routinely acquired at the end of an observation to verify the pointing of the spacecraft. The FES picture is taken with the target either in the aperture, or at a reference point 1 arcmin from the HIRS aperture towards the center of the FES field of view, depending on instructions contained in the phase 2 observing proposal. By convention, this exposure always has exposure ID 701. Other FES images are usually obtained throughout the observation (typically one per orbit) for guide star acquisitions. These images are also stored and are available to the observer. They have observation IDs and exposure IDs corresponding to the those used for the FUV exposure.
The active image area of the FES CCD images is 512 x 512 pixels, with pixel size of 2.5 arcsec. An unbinned image has 8 rows and 8 columns of overscan, resulting in a 520 x 520 raw image. Binned images also have binned overscan, so a 2 x 2 binned image has size 260 x 260 pixels. The raw data is stored as unsigned integers in FITS extensions.
The FES images are processed by stripping the overscan areas, subtracting the bias level and dividing by the flat field. The calibrated data values are 4-byte floating-point numbers. The resulting image is 512 x 512.
In February 2002, the decision was made to correct the science data for pointing stability (jitter) in the pipeline. The initial jitter correction software was first installed in the operational pipeline with CALFUSE 2.2.0 in early August. The jitter correction requires time-resolved engineering telemetry: the jitr file, and its progenitor, the hskp file.
Through 2001, FUSE jitter under guide star control was < 0.5" RMS, well within spec. In late 2001, two of the four reaction wheels (RWAs) used to control the pointing stopped functioning. In January 2002, the ACS software was updated to use the magnetic torquer bars and the geomagnetic field to control the pointing around one axis (called the anti-symmetric axis, or A-axis). Control around the other two axes was handled by the two remaining RWAs (the symmetric axis, or S-axis, and the roll axis). The A-axis and the S-axis are rotated by 45 degrees from the original instrument X and Y axes.
The amount of magnetic torque available at any given time depends on the satellite's orientation within the geomagnetic field, which in turn depends on the target location in the sky and the orientation of the satellite orbit with respect to the geomagnetic field. As a result, the available torque changes both annually and during the course of an orbit. Annual changes affect schedulability: targets are scheduled only if magnetic torque authority is sufficient during each orbit. Orbital changes in available magnetic torque affect the pointing stability at a given target. These orbital drifts can affect the quality of the science data.
In September 2002, Mission Planning began to schedule observations with gaps in torque authority, to increase the fraction of the sky available. Torque authority gaps are times when the predicted amount of torque available for A-axis control is less than the minimum required to compensate for gravity gradient torque. During these gaps, the spacecraft pointing can change rapidly and by large amounts, up to 20 degrees in a space of 10 minutes, with a return to nominal pointing requiring another 10-30 minutes. Exposures can be affected by these jumps in pointing.
"Jitter" is the high-frequency, small amplitude deviation of the actual pointing around the commanded pointing. Jitter is caused by corrections to drift made by the pointing control feedback loop and by noise in the system. The timescale is seconds, the range is approximately 1 arcsecond, and the RMS is approximately 0.3 arcsec in the science X and Y axes. Normal jitter does not degrade spectral resolution.
"Drift" is the low-frequency change in measured spacecraft pointing. In 2-wheel mode, drift around the A-axis on the time scale of half an orbit with a range of 2-20 arcsec was caused under normal conditions by insufficient compensation for gravity gradient torques. After studying this "orbital variation" for several months, OSC determined that it could be controlled by changing the integral term of the A-axis controller in the control law. The new term was loaded on the spacecraft on September 3 2000, and it works quite nicely. Orbital variation can degrade the resolution, and can cause the target to leave the aperture for part of the exposure.
Finally, the large, rapid pointing deviations caused by torque authority gaps can cause the target to leave the aperture for part of an exposure.
The hskp files contain time resolved engineering data which is useful to the FUSE Science Operations Team for diagnosing instrument or pointing problems during science exposures. The bulk of the data provide inputs for calculation of jitter and evaluation of pointing quality.
The hskp files are produced by the FUSE OPUS HSKP process from the hkn files of telemetry extracted from the FUSE telemetry database for each science exposure (TTAG or HIST). The time period included in the telemetry files begins 0.5-1.0 minutes before the science exposure and ends 1.0 minute after the science exposure ends, to ensure that all the necessary information is included. The time duration in seconds of the telemetry included in the file can be obtained from the value of NAXIS2 in the extension 1 header. (See also the EXP_DUR keyword below.)
The data are stored in a binary table. The fields of the table are named with the mnemonic of the telemetry parameter in the engineering telemetry database. Telemetry values are placed in 1-second bins according to the time at which the parameter is reported by the spacecraft (in units of MJD days). Due to lack of precision, the bins are not precisely 1 second long. Nominal update rates range from once per second to once every 16 seconds. Any parameter not reported in a given second is given the value of "-1" in the hskp file. Occasionally, parameters are reported twice in a second, in which case the final value reported during that second is placed into the hskp file.
Each type of parameter is checked for telemetry gaps based on its nominal update period. Gaps are reported in the trailer file for the exposure according to the source hkn telemetry file. The contents of each file and update rate are listed in Table 6.7-1.
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Table 6.7-1. Contents of hkn files
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The telemetry parameters included in the hskp data are listed in the example below. Detailed definitions can be found in other documentation available to the FUSE Science Operations team. The I_CENTTBL_* parameters have not been populated in the hskp files since 22 March 2002. These parameters will be removed from the hskp files in some future version of the OPUS pipeline.
XTENSION= 'BINTABLE' / Binary image extension
BITPIX = 8 / Bits per pixel
NAXIS = 2 / Number of data axes
NAXIS1 = 320 / Size of the first axis
NAXIS2 = 4199 / Size of the second axis
PCOUNT = 0 / Size of special data area
GCOUNT = 1 / One data group (required keyword)
TFIELDS = 67 / Number of fields in each row
EXTNAME = 'ENG HOUSEKEEPING ' / Name of this binary table extension
EXTVER = 1 / Extension version number
BUNIT = 'UNITLESS ' / Brightness units
TTYPE1 = 'MJD ' / Engineering time (Modified Julian Date)
TUNIT1 = 'days ' / units for MJD
TFORM1 = 'D ' / double precision
TTYPE2 = 'AATTMODE ' / Attitude estimation mode 5=Fine 4=Coarse
TFORM2 = 'J ' / integer value
TTYPE3 = 'AATTQECI2BDY_1 ' / ACS estimated body quaternion X component
TFORM3 = 'D ' / double precision
TTYPE4 = 'AATTQECI2BDY_2 ' / ACS estimated body quaternion Y component
TFORM4 = 'D ' / double precision
TTYPE5 = 'AATTQECI2BDY_3 ' / ACS estimated body quaternion Z component
TFORM5 = 'D ' / double precision
TTYPE6 = 'I_FPD_Q_ECI2BDY_1 ' / IDS FPD measured quaternion X comp
TFORM6 = 'D ' / double precision
TTYPE7 = 'I_FPD_Q_ECI2BDY_2 ' / IDS FPD measured quaternion Y comp
TFORM7 = 'D ' / double precision
TTYPE8 = 'I_FPD_Q_ECI2BDY_3 ' / IDS FPD measured quaternion Z comp
TFORM8 = 'D ' / double precision
TTYPE9 = 'I_FPD_Q_ECI2BDY_CM' / IDS commanded ECI to body quat X comp
TFORM9 = 'D ' / double precision
TTYPE10 = 'I_FPD_Q_ECI2BDY_CM' / IDS commanded ECI to body quat Y comp
TFORM10 = 'D ' / double precision
TTYPE11 = 'I_FPD_Q_ECI2BDY_CM' / IDS commanded ECI to body quat Z comp
TFORM11 = 'D ' / double precision
TTYPE12 = 'I_FPD_EXP_DURATION' / FPD exposure duration (seconds)
TFORM12 = 'E ' / floating point
TTYPE13 = 'I_FPD_MEAS_Q_VALID' / FPD measured quaternion valid flag: 0,1
TFORM13 = 'J ' / integer value
TTYPE14 = 'I_FPD_NEW_CMD_Q ' / FPD new commanded quaternion flag:(NotNew,New)
TFORM14 = 'J ' / integer value
TTYPE15 = 'I_FPD_ROLLINFPRSNT' / FPD roll information present: 0,1 (No,Yes)
TFORM15 = 'J ' / integer value
TTYPE16 = 'I_FPD_STARFLD_KNWN' / Tracking on Unknown/Known stars:(Unkn,Known)
TFORM16 = 'J ' / integer value
TTYPE17 = 'I_FPD_TRACKING_ON ' / FPD tracking on: 0,1 (Off,On)
TFORM17 = 'J ' / integer value
TTYPE18 = 'CENTROID1_STATUS ' / guidestar 1 Unused,Unver,Ver&Used,VerNotUsed
TFORM18 = 'J ' / integer value
TTYPE19 = 'CENTROID2_STATUS ' / guidestar 2 Unused,Unver,Ver&Used,VerNotUsed
TFORM19 = 'J ' / integer value
TTYPE20 = 'CENTROID3_STATUS ' / guidestar 3 Unused,Unver,Ver&Used,VerNotUsed
TFORM20 = 'J ' / integer value
TTYPE21 = 'CENTROID4_STATUS ' / guidestar 4 Unused,Unver,Ver&Used,VerNotUsed
TFORM21 = 'J ' / integer value
TTYPE22 = 'CENTROID5_STATUS ' / guidestar 5 Unused,Unver,Ver&Used,VerNotUsed
TFORM22 = 'J ' / integer value
TTYPE23 = 'CENTROID6_STATUS ' / guidestar 6 Unused,Unver,Ver&Used,VerNotUsed
TFORM23 = 'J ' / integer value
TTYPE24 = 'I_AT_CMD_ATT ' / IDS at commanded attitude: 0,1 (No,Yes)
TFORM24 = 'J ' / integer value
TTYPE25 = 'I_NOBS ' / No. guide stars Ver&Used at this time
TFORM25 = 'J ' / integer value
TTYPE26 = 'I_GMODE ' / IDS guide mode Idle,Slew,Img,UnIDtrk,IDtrk,MvT
TFORM26 = 'J ' / integer value
TTYPE27 = 'I_NEW_QCMD_FLAG ' / FPD contains new commanded quaternion: 0,1
TFORM27 = 'J ' / integer value
TTYPE28 = 'I_NEW_QMEAS_FLAG ' / FPD contains new measured quaternion: 0,1
TFORM28 = 'J ' / integer value
TTYPE29 = 'I_TM_OUTPUT_RATE ' / Telem subset 0=A 5=B 7=Inval others:memorydump
TFORM29 = 'J ' / integer value
TTYPE30 = 'I_CENTTBL_TRK_LOCK' / centroids being tracked successfully: 0,1
TFORM30 = 'J ' / integer value
TTYPE31 = 'I_CENTTBL_FRM_VAL ' / current frame of centroids valid: 0,1 (No,Yes)
TFORM31 = 'J ' / integer value
TTYPE32 = 'I_CENTTBL_S1_BDCNT' / no. consec. frames invalid this centroid
TFORM32 = 'J ' / integer value
TTYPE33 = 'I_CENTTBL_S2_BDCNT' / no. consec. frames invalid this centroid
TFORM33 = 'J ' / integer value
TTYPE34 = 'I_CENTTBL_S3_BDCNT' / no. consec. frames invalid this centroid
TFORM34 = 'J ' / integer value
TTYPE35 = 'I_CENTTBL_S4_BDCNT' / no. consec. frames invalid this centroid
TFORM35 = 'J ' / integer value
TTYPE36 = 'I_CENTTBL_S5_BDCNT' / no. consec. frames invalid this centroid
TFORM36 = 'J ' / integer value
TTYPE37 = 'I_CENTTBL_S6_BDCNT' / no. consec. frames invalid this centroid
TFORM37 = 'J ' / integer value
TTYPE38 = 'I_CNFGCENT_FAIL_ST' / ConfigCents status 0=ok 1=configure 2,3,4=err
TFORM38 = 'J ' / integer value
TTYPE39 = 'I_MT_FAIL_STAT ' / MovingTarget status 0=ok 2=config 1,3,4,5=err
TFORM39 = 'J ' / integer value
TTYPE40 = 'I_PO_FAIL_STAT ' / Pointing offset status 0=ok 1,2=err
TFORM40 = 'J ' / integer value
TTYPE41 = 'I_PU_FAIL_STAT ' / peakup status 0=ok 1,2,3,4=err
TFORM41 = 'J ' / integer value
TTYPE42 = 'I_TC_FAIL_STAT ' / target correction 0=ok 1=config 2,3,4=err
TFORM42 = 'J ' / integer value
TTYPE43 = 'I_UTRACK_FAIL_STAT' / Unidentified track ok,config,star_sel,reject
TFORM43 = 'J ' / integer value
TTYPE44 = 'I_IDS_IGNORED ' / ACS uses/ignores IDS FPDs: 0,1 (Use,Ignore)
TFORM44 = 'J ' / integer value
TTYPE45 = 'AQECI2BDYCMD_1 ' / ACS commanded quaternion X axis: range (-1,1)
TFORM45 = 'D ' / double precision
TTYPE46 = 'AQECI2BDYCMD_2 ' / ACS commanded quaternion Y axis: range (-1,1)
TFORM46 = 'D ' / double precision
TTYPE47 = 'AQECI2BDYCMD_3 ' / ACS commanded quaternion Z axis: range (-1,1)
TFORM47 = 'D ' / double precision
TTYPE48 = 'I_DET1CFEA ' / DET1 Counter Fast Events A
TFORM48 = 'J ' / integer value
TTYPE49 = 'I_DET1CFEB ' / DET1 Counter Fast Events B
TFORM49 = 'J ' / integer value
TTYPE50 = 'I_DET2CFEA ' / DET2 Counter Fast Events A
TFORM50 = 'J ' / integer value
TTYPE51 = 'I_DET2CFEB ' / DET2 Counter Fast Events B
TFORM51 = 'J ' / integer value
TTYPE52 = 'I_DET1CAIA ' / DET1 Counter Active Image A
TFORM52 = 'J ' / integer value
TTYPE53 = 'I_DET1CAIB ' / DET1 Counter Active Image B
TFORM53 = 'J ' / integer value
TTYPE54 = 'I_DET2CAIA ' / DET2 Counter Active Image A
TFORM54 = 'J ' / integer value
TTYPE55 = 'I_DET2CAIB ' / DET2 Counter Active Image B
TFORM55 = 'J ' / integer value
TTYPE56 = 'I_DET1CSICA ' / DET1 Counter SiC A
TFORM56 = 'J ' / integer value
TTYPE57 = 'I_DET1CSICB ' / DET1 Counter SiC B
TFORM57 = 'J ' / integer value
TTYPE58 = 'I_DET2CSICA ' / DET2 Counter SiC A
TFORM58 = 'J ' / integer value
TTYPE59 = 'I_DET2CSICB ' / DET2 Counter SiC B
TFORM59 = 'J ' / integer value
TTYPE60 = 'I_DET1CLIFA ' / DET1 Counter LiF A
TFORM60 = 'J ' / integer value
TTYPE61 = 'I_DET1CLIFB ' / DET1 Counter LiF B
TFORM61 = 'J ' / integer value
TTYPE62 = 'I_DET2CLIFA ' / DET2 Counter LiF A
TFORM62 = 'J ' / integer value
TTYPE63 = 'I_DET2CLIFB ' / DET2 Counter LiF B
TFORM63 = 'J ' / integer value
TTYPE64 = 'I_DET1HVBIASAST ' / DET1 MCP-A High Voltage Bias setting
TFORM64 = 'J ' / integer value
TTYPE65 = 'I_DET1HVBIASBST ' / DET1 MCP-B High Voltage Bias setting
TFORM65 = 'J ' / integer value
TTYPE66 = 'I_DET2HVBIASBST ' / DET2 MCP-B High Voltage Bias setting
TFORM66 = 'J ' / integer value
TTYPE67 = 'I_DET2HVBIASAST ' / DET2 MCP-A High Voltage Bias setting
TFORM67 = 'J ' / integer value
END
The jitr data files contain time resolved data of the spacecraft pointing relative to the commanded pointing. These data are derived from the hskp data (see above). The file is a binary table with 1-second time resolution.
TIME is the time in seconds since the Reference time, defined as the Science Exposure Start time (DATEOBS and TIMEOBS keywords in the jitr header).
DX is the offset of the target from the commanded position in arcseconds along the science instrument X axis. DY is the offset of the target from the commanded position in arcseconds along the science instrument Y axis.
TRKFLG is a flag indicating the quality of the tracking for that data point. The values are 5, 4, 3 and -999, where "5" is the best quality. A value of "5" means stars are available for tracking and the guiding solution computed from the stars (the FPD measured quaternion) is being used by the ACS. A value of "4" means stars are available but the guiding solution computed from the stars is not being used by the ACS. A value of "3" means no stars are available for a guiding solution, and the ACS is using other information to control the pointing. A value of "-999" means that no data is available to judge the tracking quality. TRKFLG values indicate which of the measured quaternions from the hskp were used to compute DX and DY. TRKFLG values of "5" and "4" indicate that hskp FPD measured quaternions were used to compute DX and DY. A TRKFLG value of "3" means the hskp ACS estimated body quaternions were used to compute DX and DY.
XTENSION= 'BINTABLE' / binary table extension BITPIX = 8 / 8-bit bytes NAXIS = 2 / 2-dimensional binary table NAXIS1 = 14 / width of table in bytes NAXIS2 = 4199 / number of rows in table PCOUNT = 0 / size of special data area GCOUNT = 1 / one data group (required keyword) TFIELDS = 4 / number of fields in each row TTYPE1 = 'TIME ' / label for field 1 TFORM1 = '1J ' / data format of field: 4-byte INTEGER TUNIT1 = 'seconds ' / physical unit of field TTYPE2 = 'DX ' / label for field 2 TFORM2 = '1E ' / data format of field: 4-byte REAL TUNIT2 = 'arcsec ' / physical unit of field TTYPE3 = 'DY ' / label for field 3 TFORM3 = '1E ' / data format of field: 4-byte REAL TUNIT3 = 'arcsec ' / physical unit of field TTYPE4 = 'TRKFLG ' / label for field 4 TFORM4 = '1I ' / data format of field: 2-byte INTEGER TUNIT4 = ' ' / physical unit of field EXTNAME = 'jitter ' / name of this binary table extension END
The FITS keywords are organized into sections in the header. The DATA DESCRIPTION KEYWORDS, PROPOSAL INFORMATION, and TARGET INFORMATION sections in the hskp and jitr file headers contain the same keywords as those sections in the raw and calibrated files.
DATA DESCRIPTION KEYWORDS
TELESCOP= 'FUSE ' / Telescope used to acquire data
INSTRUME= 'FUV ' / Instrument in use (one of FUV, FESA, or FESB)
ROOTNAME= 'C1280102001 ' / Rootname of the observation set (ppppttooeee)
FILENAME= 'C1280102001hskpf.fit ' / Filename
FILETYPE= 'HOUSEKEEPING ' / Type of data found in data file
EXP_STAT= 0 / Science Data Processing status (0=good)
PROPOSAL INFORMATION
PRGRM_ID= 'C128 ' / Program ID from data header
TARG_ID = '01 ' / Target ID from data header
SCOBS_ID= '02 ' / Observation ID from data header
EXP_ID = '001 ' / Exposure ID from data header
OBS_ID = '02 ' / Observation ID from proposal database
PR_INV_L= 'Brown ' / Last name of principal investigator
PR_INV_F= 'Thomas M. ' / First name of principal investigator
TARGET INFORMATION
TARGNAME= 'NGC221 ' / Target name on proposal
RA_TARG = 10.674458 / [deg] Right ascension of the target (J2000)
DEC_TARG= 40.865889 / [deg] Declination of the target (J2000)
APER_PA = 308.344391 / [deg] Position angle of Y axis (E of N)
EQUINOX = 2000.0 / Equinox of celestial coord. system
ELAT = 3.299589187390E+01 / [deg] Ecliptic latitude
ELONG = 2.761632789104E+01 / [deg] Ecliptic longitude
GLAT = -2.197586507735E+01 / [deg] Galactic latitude
GLONG = 1.211515483814E+02 / [deg] Galactic longitude
The ENGINEERING HOUSEKEEPING TIME keywords specify the start time of the engineering telemetry contained in the file. These keywords are populated from the engineering housekeeping data, and so are measured quantities. The TIME_ENG keyword value is normally about one-half to one minute before the actual start time of the science exposure.
ENGINEERING HOUSEKEEPING TIME
DATE_ENG= '2002-08-26 ' / start date (yyyy-mm-dd)
TIME_ENG= '18:17:38 ' / start time (hh:mm:ss)
The SUMMARY EXPOSURE INFORMATION keywords contain a mix of measured and predicted values. The actual start time of the science exposure is contained in DATEOBS, TIMEOBS, and EXPSTART - these are measured values. The exposure duration, however, is the predicted value populated from the MPDB, and so EXPTIME is not a measured quantity.
SUMMARY EXPOSURE INFORMATION
DATEOBS = '2002-08-26 ' / UT date of start of exposure (yyyy-mm-dd)
TIMEOBS = '18:18:36 ' / UT start time of exposure (hh:mm:ss)
EXPSTART= 52512.762917 / Exposure start time (Modified Julian Date)
EXPTIME = 3811.0 / [sec] Exposure duration -- calculated
The ASSOCIATION KEYWORDS is the final section in the primary header for both the hskp and jitr files. For the hskp file this set of keywords is minimal, containing only the information to link this file to the association.
ASSOCIATION KEYWORDS
ASN_ID = 'C1280102000 ' / Unique identifier assigned to association
ASN_TAB = 'C1280102000asn.fit ' / Name of the association table
ASN_MTYP= 'EXPOSURE ' / Role of the exposure in the association
END
In the jitr file, the ASSOCIATION KEYWORDS section also includes the specific jitter keywords (we expect to move them to a JITTER KEYWORDS section in a future version of the pipeline).
The jitter data are computed from data in the housekeeping file. Most of the jitter keywords reflect the results of that analysis, and are intended as indicators of the overall quality of the pointing during the science exposure.
JIT_STAT is set to 0 if jitter was computed, and it is set to 1 if there were problems computing the jitter.These problems include the following. (a) The absence of a housekeeping file containing the raw data on which the jitter analysis is based. (b) Problems with the tabulated starting time of the exposure. These include an exposure start time which is more than 1 day after the start of the jitter data and an exposure start which is earlier than the first tabulated jitter time. Typically, the jitter data starts 30 seconds before the start of the exposure. (c) When the reference pointing location could not be determined from the data.
EXP_DUR is the duration of telemetry in the jitr file. The telemetry in the housekeeping data is specified to start 0.5-1.0 minutes before the start of the associated science exposure, and to end 1 minute after the end of the associated science exposure. Thus, the jitter EXP_DUR should be 90-120 seconds longer the science exposure time EXPTIME. N.B. As of this writing (November 2002), EXP_DUR is set equal to EXPTIME, and EXPTIME is the predicted exposure duration from the MPDB. This problem will be fixed in a future release of CALFUSE. For now, the duration of the telemetry in the jitr file can be obtained from the NAXIS2 value in the hskp or jitr data extension header.
FINE_GDE is the fraction of EXP_DUR when the spacecraft attitude control system (ACS) was in fine guidance mode, using known or unknown stars to control the pointing. A value less than about 0.95 indicates a problem.
COARSGDE is the fraction of EXP_DUR when the ACS was in coarse guidance mode.
NOGDEINF is the fraction of EXP_DUR for which there was no ACS guidance mode information available.
GSn_USED is the fraction of EXP_DUR when guide star n was contributing to the guiding solution computed by the IDS. Normally, 3 to 6 guide stars are scheduled for an observation. Rarely as few as 1 or 2 are scheduled. Values of 0 are assigned to any star not scheduled to be used. Any star with a severe problem preventing use may have a value of 0 for this keyword. In general, these keywords indicate a problem if
(1) the keyword value differs significantly for the guide stars in use during a single exposure; (2) a lower-numbered guide star is 0 while a higher-numbered guide star is non-zero; (3) the number of guide stars in use changes among the jitr files in the same observation.
KNOWNTRK is the fraction of EXP_DUR when the IDS was using scheduled known guide stars to compute a guiding solution. In known tracking, the pointing position is known relative to the stars, and the target should be at the desired place in the aperture.
UNKWNTRK is the fraction of EXP_DUR when the IDS was using unknown stars to compute a guiding solution. In unknown tracking, stars are used to control the pointing of the spacecraft, but the identity of these stars is not known. The target may or may not be at the desired place in the aperture, depending on preceding events.
GS_INUSE is the fraction of EXP_DUR when the IDS was using stars for guiding, whether Known or Unknown.
SLEWFLG is the fraction of EXP_DUR when the spacecraft was executing a commanded slew. A non-zero value may indicate a guiding problem during the exposure, except in certain cases. A non-zero value is not a problem for science exposures for which scans were purposely executed, including M112's, M114's, and certain other calibration observations. There are two other cases where a non-zero value may not be a problem. Because the jitr "exposure" starts 30-60 seconds before the science exposure, a slew that completed prior to the science exposure may be included in the housekeeping data. Certain science exposures are more likely to be affected by this mis-match: the first exposure following the guide star acquisition; the first exposure following a peakup.
POSAVG_X is the average of DX during the exposure (DX is defined above).
POSAVG_Y is the average of DY during the exposure (DY is defined above).
X_JITTER is the standard deviation of DX in the telemetry file. This value gives some indication of the orbital variation.
Y_JITTER is the standard deviation of DY in the telemetry file. This value gives some indication of the orbital variation.
X_JIT_5M is the standard deviation of DX during a 5-minute segment near the end of the telemetry file. This value is the jitter for the exposure.
Y_JIT_5M is the standard deviation of DY during a 5-minute segment near the end of the telemetry file. This value is the jitter for the exposure.
X_JITLRG is fraction of EXP_DUR for which DX is more than 2-sigma from the average
Y_JITLRG is fraction of EXP_DUR for which DY is more than 2-sigma from the average.
ASSOCIATION KEYWORDS
ASN_ID = 'C1280102000 ' / Unique identifier assigned to association
ASN_TAB = 'C1280102000asn.fit ' / Name of the association table
ASN_MTYP= 'EXPOSURE ' / Role of the exposure in the association
JIT_STAT= 0 / status of jitter data is good
EXP_DUR = 3.811000E+03 / exposure duration
FINE_GDE= 9.997376E-01 / fraction of exp in fine guide
COARSGDE= 0.000000E+00 / fraction of exp in coarse guide
NOGDEINF= 0.000000E+00 / fraction of exp with no guiding info
GS1_USED= 9.997376E-01 / fraction of exp using guide star 1
GS2_USED= 9.997376E-01 / fraction of exp using guide star 2
GS3_USED= 9.997376E-01 / fraction of exp using guide star 3
GS4_USED= 9.997376E-01 / fraction of exp using guide star 4
GS5_USED= 0.000000E+00 / fraction of exp using guide star 5
GS6_USED= 0.000000E+00 / fraction of exp using guide star 6
NGS_USED= 4 / number of guide stars used
KNOWNTRK= 9.997376E-01 / fraction of exp tracking on known stars
UNKWNTRK= 0.000000E+00 / fraction of exp tracking on unknown stars
GS_INUSE= 9.997376E-01 / fraction of exp tracking on guide stars
SLEWFLG = -1 / slew commanded during obs (if > 0)
POSAVG_X= 9.458315E-01 / [arcsec] mean DX during exposure
POSAVG_Y= 9.576777E-01 / [arcsec] mean DY during exposure
X_JITTER= 1.393764E+00 / [arcsec] sigma of DX during exposure
Y_JITTER= 1.401616E+00 / [arcsec] sigma of DY during exposure
X_JIT_5M= 3.399618E-01 / [arcsec] sigma of DX during last 5 min of exp
Y_JIT_5M= 3.696457E-01 / [arcsec] sigma of DY during last 5 min of exp
X_JITLRG= 3.906057E-02 / frac of DX more than 2-sigma from POSAVE_X
Y_JITLRG= 3.337454E-02 / frac of DY more than 2-sigma from POSAVE_Y
END
The processing steps which are performed on the raw FUV data to produce extracted, calibrated spectra are described in detail in the CalFUSE Pipeline Reference Guide.
In order to read and display FUSE images and spectra, one must have software to read FITS binary table files as well as 2D images which are located in FITS extensions. All FUSE data products adhere to the FITS standard. Those wishing to write their own data reduction or display software in C or FORTRAN are encouraged to use the cfitsio package of FITS file routines available at the FITSIO website: