The FUSE Observer's Guide Appendix D: Acquisition Types Discussion Version 2.0, May 10, 2000

WARNING!!! Some information in this document may be dated!!

This appendix was formed from material cut out of the FUSE Observer's Guide, ver. 1.2. It contains extra discussion that was deemed "optional" for Cycle 2 observers and beyond, but may still be of interest. Some of this information has been duplicated and updated in the FUSE Observer's Guide, Version 2.0. For Any material shown here and in Version 2.0, the information in the main document is more current and should be used.

Outdated material in this appendix will be updated only as time permits.

D.1 Target Acquisition

The process of acquiring a target and placing it in the desired spectrograph aperture varies considerably in operational difficulty, depending largely on the aperture chosen (cf. section 2.3 above), the nature of the target, and (to a lesser extent) the actual performance of the satellite itself. You need to know enough about the acquisition process to make intelligent decisions about the aperture to use and when your targets are likely to need an offset star for a good target acquisition. The Phase 2 Instructions provide detailed instructions on how to specify offset stars.

We first discuss the general processes involved in target acquisitions and then discuss the different cases we are likely to encounter in real operations. The simplest case is that of a fixed (non-moving) point source. Acquisitions of extended objects, optically bright objects and moving targets present their own individual problems and are discussed below.

D.1.1 General Field Acquisition

The accuracy of the slew (<10 arcmin [1 sigma] even for long slews) will be good enough to place the target within or near the FES field of view, which is roughly 19.5 arcmin square. When the target becomes unocculted by the earth, the IDS will send a previously stored command to the FES to take a short (1-5 sec) exposure. In response, the FES will read out the CCD and send the digital image to the IDS.

After receiving the FES image, a process in the IDS will be invoked to locate the brightest stars in the image. These star locations will be used for field identification. Several bright stars will also be used for guiding while the IDS performs the field identification. The IDS determines the boresight pointing by comparing the star locations from the image with a table of star locations (from the HST Guide Star Catalog) produced by FUSE Mission Planners and uplinked for this purpose.

After sending the measured boresight pointing to the spacecraft ACS, the ACS will autonomously slew the satellite to correct the difference between the commanded and measured pointing positions. This will result in the target being placed in or near the desired spectrograph entrance aperture. The positions of selected guide stars are then centroided every second with the FES. Positions are sent to the IDS, which combines these positions (in a flux weighted manner) to produce a measured pointing. The IDS then sends this measured pointing value to the ACS, which in turn acts on this information to maintain pointing stability.

D.1.2 Types of Target Acquisitions

1. The four channels may need to be aligned to maximize throughput (most probably only if the HIRS aperture has been selected), and/or
2. The target must be autonomously centered in all four spectrograph entrance slits (which are located on the four FPAs). In the limit where channel alignment is stable, the target centering simplifies to centering the target in the aperture visible to the active FES.

Briefly, the different target acquisition types are as follows:

1. FUV PEAK-UP: The standard acquisition mode in cases where channel alignment and/or careful centering of the target in the slit are required. (Assumes target is bright enough in FUV for this to work; see below.)
2. FES Target Acq: Target is sufficiently bright visually (but not too bright) to be seen and centroided by the FES. Should work for targets with V magnitudes between about 8 and 14. Assumes no channel alignment is needed.
3. FES Target Acq plus FUV PEAK-UP: Target is sufficiently bright visually to be seen and centroided by the FES, and most of the positional uncertainty is removed by the FES Target Acq. Additionally, source is bright enough to permit a PEAK-UP (for optimum centering in the narrow slit, for instance).
4. FES Guide Star Acq: Target is too faint or diffuse to be centroided directly, but its position is well known with respect to the selected guide stars. FES acquires selected guide stars, positions them, and assumes the target is in the aperture of choice.
5. Blind Offset from within FES FOV: Used in certain cases where a special offset star is identified for use in the acquisition process. Offset stars of this type are expected to be very close to the target position, and certainly within the FES FOV. An FES Target Acq is done on the offset star, but then the "science target" position is placed in the selected aperture. (Note: this kind of offset star is NOT specified on the Phase 2 target list, but is handled separately.)

There may also be cases where a separate offset star (i.e., outside the FES FOV of the target) must be acquired first. In this case, the offset star is acquired using one of the standard cases above and then the science target may be accquired with another.

D.1.3 Target Centering in the Aperture

After the small angle maneuver described above, the target will be within several arcseconds of the intended aperture. The method of centering your target depends on which aperture is being used and what the properties of the target are. For the most stringent cases, the channel alignment (if needed) and target centering both occur through the use of a single operational procedure known as a FUV PEAK-UP. To perform the PEAK-UP, the pointing of the satellite is dithered in a "step and dwell" pattern perpendicular to the slit (in the X direction) to find the maximum FUV signal from the target (through each aperture if channel alignment is needed). The dwell time at each step in the process depends on the expected FUV flux from the target. If all four channels are aligned, then the signal will be maximized through each slit at a common pointing. The data obtained in this peak-up process are used by the IDS to determine how much to move the FPAs and slew the satellite to center the target in all four channels. Once this step is completed, an FUV observation can begin.

PEAK-UP Assumptions and Usage:

We will not know how often PEAK-UPs are required until after launch. For instance, issues about the stability of the channel alignment with time or with thermal conditions, whether channel alignments or PEAK-UPs are required in the MDRS slit, etc., are things that cannot be specified with certainty at this time. Hence, for pre-mission purposes we need to make certain assumptions about when FUV PEAK-UPs will be needed. Thus:

   1. Assume PEAK-UPs will be required, for channel alignment and/or for 
      target centering, whenever a point source is to be placed in the HIRS 
      aperture.

   2. Assume PEAK-UPs will NOT be required for the MDRS aperture. Target
      centering will be accomplished via FES Target Acq.

   3. Assume PEAK-UPs will never be required for the LWRS aperture.

   4. Assume an average flux across a given channel of 4.0E-14 
      ergs/cm^2-s-Å to be the MINIMUM required for a peak-up source.  
      This applies both to an object to be peaked-up on, or a supplied 
      offset star.

Note that PEAK-UPs are only required for point sources being placed in the narrow aperture (HIRS). If your source is too faint for a confident PEAK-UP and you really need the HIRS aperture, you MUST supply an offset star with reasonable FUV fluxes near your target (within about 2 degrees, but the closer the better). If no good offset stars are available, or if you don't absolutely need the HIRS aperture, switch to the MDRS aperture. If your target is faint, you should really consider using the MDRS aperture anyway, since the slit losses in the HIRS aperture are about 35%.

For PEAK-UPs to work, we must determine a dwell time for the peak-up steps to reach a minimum expected S/N=10 in the least sensitive channel. This will depend also on the shape of the source spectrum. The flux information you provide in Phase 2 is used to assess this situation. Using the on-line Count Rate Tool and specifying "constant spectrum" sources with the indicated fluxes, we see the following:

        DWELL TIME FOR S/N=10 (in sec)

    Flux      SiC1    SiC2    LiF1    LiF2
   --------  ------  ------  ------  ------
   1.0E-14   743.27  760.30  170.29  197.56
   2.0E-14   224.29  229.03   58.83   67.20  
   4.0E-14    75.31   76.74   22.84   25.70  <--(Stated Limiting flux for
   6.0E-14    42.02   42.76   13.76   15.38         FUV PEAK-UP)
   8.0E-14    28.44   28.92    9.77   10.88 
   1.0E-13    21.28   21.63    7.56    8.39 
   5.0E-13     3.31    3.36    1.34    1.48 

*Assumes constant spectrum source at indicated flux level, HIRS slit, 
a dark rate of 1.0 counts/cm^2-s, and 10.00 kRayleigh of airglow 
(Ver. 1.2 of Count Rate Tool).

As a guideline, we have stated an average flux of ~4 × 10^-14   ergs cm^-2s^-1 Å^-1 in any channel as the limiting flux for successful PEAK-UPs. However, as shown above, even at this level, the dwell times per step are longer than 1 minute in the SiC channels, and it remains to be demonstrated (on-orbit) whether performance will allow this or not. Also notice that the dwell time is a very steep function at the lowest allowed fluxes. Hence, any uncertainties in the fluxes at short wavelengths could potentially cause failures in the PEAK-UPs in the SiC channels. We encourage users to be conservative in suggesting PEAK-UPs for targets at or near this limit. For fainter targets, the investigator should specify an "offset" star within 2° of the target that has sufficient FUV brightness for a PEAK-UP (see Phase 2 Instructions for details). The FUV PEAK-UP will be performed on this offset star, and then a blind offset will be performed to place the target in the aperture (or near the aperture for subsequent slit centering if needed).

The other common acquisition mode, used either by itself or in conjunction with a PEAK-UP, is the FES Target Acq. In this case, the target is in a range of visual magnitude that allows the FES to centroid it directly. This has the advantage of removing any residual coordinate uncertainties with respect to the guide stars, allowing the target to be placed directly into the aperture of choice. (For the HIRS aperture, however, a PEAK-UP may still be needed for channel alignment or improved centering.) The exact visual magnitude range over which this technique will be effective will be determined after launch; we assume this range to be V = 8 - 14 pre-launch.

D.1.4 Optically Very Bright Targets

Targets that are very bright in the optical, but faint in the FUV (such as late type stars), present a special observational challenge. These targets will be very saturated even in a short FES exposure (without the neutral density filter), and may also have large amounts of scattered light, possibly making it difficult to perform field acquisition and PEAK-UP. Early tests of the FES in I&T have been promising; it appears that scattered light from a bright star may only be a problem for stars or planets with V<1 (i.e., Procyon or Jupiter). However, for pre-launch planning purposes we are using a more conservative value of V<3 for when an offset star will be needed. For these targets, we will perform the field acquisition step on a nearby offset field which does not include the bright target. If channel co-alignment is required, then this offset field should include a UV bright star that will support the FUV PEAK-UP process. Next, the satellite will be slewed to place the bright target in the desired slit. This will be a blind offset, and its accuracy will depend on the accuracy of the target coordinates in the guide star frame of reference.

For targets in the range V= 3 to 8, the target itself will be saturated in the FES, but we expect to be able to acquire guide stars in most cases. Hence, FES Guide Star Acq will be planned with these sources unless inspection of the available guide stars shows a problem.

Another technique for observing very bright targets is to employ the neutral density (ND) filter (6 magnitudes of attenuation) in the FES. However, when the ND filter is in place, no guide stars are likely to be visible in the FES FOV except the target itself. Consequently, we would have to guide on the "spillover" light from the target (i.e., the light not transmitted through the slit). For a star, this would only work for the HIRS slit (for which 35% of the light is reflected into the FES). For a planet such as Jupiter, the HIRS or MDRS slit could be used. However, the guiding performance when using this technique is highly uncertain, and will not be known until after launch.

D.1.5 Extended Targets

Crowded fields can cause acquisition problems in one of several ways. For instance, stars with visual magnitudes comparable to your target and within about 20 arcsec in position may adversely affect the ability to perform the FES Target Acq scenario. Alternatively if the target is an FUV-bright target in an "optically" crowded field, then the real issue could be one of guide star availability. For example, take the case of a UV bright star in the center of a globular cluster such as Omega Cen. This is a very large star cluster (on the order of the size of the FES FOV), and hence it may be very difficult to find isolated stars that can be used for guiding. Situations like this must be assessed on a case-by-case basis.

Another kind of crowded field is one containing several close-by FUV bright stars. In this case, guiding is not an issue, but target acquisition is. For example, there are a couple of fairly bright early type stars only a few arcsec away from SN 1987A. Care must be taken when observing SN 1987A that the wrong star is not acquired and centered in the slit. Furthermore, since the HIRS and MDRS slits are 20 arcsec long, such observation must be performed at specific position angles to avoid these nearby stars. Investigators should alert FUSE Mission Planners of possible crowded field complications in their Phase 2 submissions.

D.1.7 Moving Targets

FUSE does not have the capability to "track" moving targets per se. Instead, knowledge of the object's ephemeris is used to predict where the target will be at some future time and how the position will change with time throughout the FUV exposure. For standard solar system objects, the ephemeris will be calculated with MOSS (a variant of the JPL-based software supported by the STScI). A table of target positions as a function of time will be uploaded to the satellite at the beginning of the observation. The satellite will orient itself to place the target in the desired slit and then make a series of small pointing offsets as a function of time to keep the target in the slit.

Observations of very bright planets, such as Jupiter, present a problem for the FES. The neutral density (ND) filter will have to be used in the FES optical path, which means that normal guide stars cannot be used for guiding (i.e., they are not bright enough to be visible when the ND filter is in place). Consequently, guiding will be performed on Jupiter itself (since it is much larger than the HIRS and MDRS slits) or on one of its moons. We do not expect that guiding performance will meet the nominal specifications for fixed targets; the requirement is pointing stability of < 5 arcsec for objects moving at rates less than 0.2 arcsec/sec. An alternative guiding scenario for Jupiter is to guide on the moon Ganymede, which has a visible brightness of V~6 and will be visible in the FES FOV even with the ND filter in place. Guiding on a moon, which itself is also moving, will be challenging and will be tested by the PI team during the first year of the mission.

Comets, whose precise positions may not be known ahead of time, require a slightly different approach. The satellite will be pointed to place the comet at some reference position near the center of the FES field of view. The FES will then provide a centroid measurement of the comet nucleus to the IDS, which will then compute an offset slew to place the comet in the spectrograph slit. The observation then proceeds as described above, with a table of offset maneuvers as a function of time being used to keep the target in the slit. The expected relative motion of the comet must therefore be known fairly well (i.e., rate and direction), even though the absolute position at some time is uncertain.

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