Introduction
The properties of the FUSE detectors have held remarkably steady since launch, but a few time-dependent effects are worth knowing about.
Changes of the Detector High Voltage
The FUSE detectors convert each ultraviolet photon into a shower of electrons, for which the detector electronics calculate the X and Y positions and the intensity, or pulse height. Prolonged exposure to photons causes the detectors to become less efficient (a phenomenon called gain sag), and the mean pulse height slowly decreases. Unfortunately, the X location of low-pulse-height photon events is systematically miscalculated by the detector electronics. As the number of low-pulse-height events increases, spectral features appear to ``walk'' across the detector. A common example is the shoulder apparent on the short-wavelength side of the Lyman beta airglow line in the LiF 1A channel. Eventually, pulse heights could become so low that real photon events are rejected by the detector electronics. To combat this problem, we raise the detector high voltage every six months or so. The higher voltage imparts more energy to the electron shower, raising the mean pulse height and reducing the number of misplaced photon events.
Here is a history of long-term changes to the detector high voltage:
Date 1A, 1B, 2A, 2B ----------- ------------------ 13-AUG-1999 129, 129, 000, 000 Detector 1 ramped up 26-AUG-1999 129, 129, 129, 102 Detector 2 ramped up 24-JAN-2001 141, 137, 137, 108 New HV levels 31-JUL-2001 147, 143, 149, 113 New HV levels 19-FEB-2002 155, 151, 161, 119 New HV levels 14-APR-2002 155, 151, 149, 119 New lower 2A level 12-DEC-2002 161, 157, 161, 124 New HV levels 16-DEC-2002 161, 157, 134, 124 New lower 2A level 03-FEB-2003 161, 157, 149, 124 2A level restored 16-AUG-2003 164, 160, 149, 126 New HV levels
The phases of this cycle (an increase in the high voltage followed by a gradual gain sag) leave their marks in the FUSE data. Data taken just before the high voltage is raised show stronger walk effects than data taken just after. While the pipeline can correct for the walk in time-tagged data, for which the pulse height of individual photon events is recorded, the correction applied to histogram data is less satisfactory.
Background events -- both scattered light and detector dark counts -- also migrate to lower pulse heights. Because dark-count events have intrinsically lower pulse heights than real photons, they are the first to drop below the low-pulse-height threshold and be lost from the data stream. The time dependence of the detector dark count complicates efforts to model the background. Because unilluminated regions of the detector suffer less gain sag, their background level may be significantly greater than that in the region of the target spectrum, leading to an over-estimate in the background flux.
As the detectors age and their total exposure time increases, gain sag will be an ongoing problem. We will periodically raise the detector voltage levels to offset this effect, but there is an upper limit to the detector voltage -- a limit that we may have reached with Detector 2A (see below).
Detector 2: February - March 2002
On 19 February 2002, the high voltage was raised on all four detector segments. On 26 March, detector 2 shut down. Attempts to raise the voltage led to another shut-down. In the following weeks, the voltage was slowly raised in an attempt to return it to the 19 February level. This led to another shut-down on 6 April. Finally, on 14 April, the voltage on segment 2A was set to the level in use before 19 February. Detector 2 data taken during this period are likely to suffer from larger-than-usual walk effects and may suffer from errors in the flux calibration.
Here's the detailed history of the detector 2 high voltage:
26-MAR-2002 05:13 Detector 2 shuts down 26-MAR-2002 16:36 Detector 2 back to full 26-MAR-2002 17:18 Detector 2 shuts down again 29-MAR-2002 09:59 2B back to full. 2A lower (134) 29-MAR-2002 14:56 Start raising 2A back to full 3-APR-2002 21:54 2A reaches 148 on its way up 4-APR-2002 14:50 2A = 150 4-APR-2002 23:21 2A = 152 5-APR-2002 14:05 2A = 154 5-APR-2002 22:23 2A = 156 6-APR-2002 07:39 Detector 2 shut down (2A = 156) 6-APR-2002 22:01 2B back to full. 2A lower (134) 14-APR-2002 17:22 2A back to new nominal level (149)What does this mean? If we assume that anything above ~148 on segment 2A can be considered as full voltage (once we've applied the appropriate time-dependent flux calibration), then the following times are the only good ones for segment 2A since 26 March:
2002:085:16:36 to 2002:085:17:18 2002:093:21:54 to 2002:096:07:39 * 2002:104:17:22 and later* During the second period, the voltage was changing. It is possible that the voltage changed during an observation. If so, then the spectrum may change from one exposure to the next -- or even during an exposure!
Segment 2B is OK for any of the following times since 26 March:
2002:085:16:36 to 2002:085:17:18 2002:088:09:59 to 2002:096:07:39 2002:096:22:01 and later
Detector 2: December 2002 - February 2003
On 8 December 2002, the high voltage was again raised on all four detector segments. Shortly afterward, detector segment 2A began to experience high-voltage current transients, which caused the detector high voltage to shut down. Repeated attempts to return the voltage to its nominal level were unsuccessful. On 16 December 2002, we decided to leave the voltage at a lower-than-nominal value while we investigated the problem. We were finally able to restore the segment 2A voltage to its pre-December value on 3 February 2003.
Calibration data indicate that LiF 2A LWRS spectra obtained during this low-voltage period suffer significant walk effects, with typical shifts of 20 to 30 pixels in the raw data. The other LiF apertures and the SiC channel of segment 2A are much less affected. The CalFUSE pipeline corrects time-tag data for walk, but the correction for histogram data is less satisfactory. Observers are urged to examine carefully LiF 2A LWRS data obtained in histogram mode during this low-voltage period.
The effects of gain sag on the detector dark-count are discussed above. These effects are quite strong for LiF 2A LWRS data obtained during this low-voltage period. As a result, CalFUSE is likely to overestimate the background for LiF 2A LWRS spectra. Both time-tag and histogram data are affected.
Repeated observations of standard stars show no significant change in the instrumental sensitivity during this low-voltage period.
Dave Sahnow has produced an appendix to this white paper with more details, including figures and sample spectra. It is titled Appendix A: Segment 2A Gain Sag.
Flux Calibration
Repeated observations of standard white-dwarf stars indicate that the instrument sensitivity held remarkably stable over the first two years of the mission. Since the middle of 2001, however, we have noticed a slow degradation in the effective area of the FUSE spectrograph. The decline is more or less wavelength independent and affects all channels at roughly the 20% level. A time-dependent version of the flux calibration that takes into account this degradation is now available. For details, see The FUSE Flux Calibration.
Background Models
Here are the dates when the various background calibration files
become valid:
Questions?
Please address questions to
fuse_support@pha.jhu.edu.
Version # MJD Date
5 50000 first calibration
6 51911 01 Jan 2001
7 52120 01 Aug 2001
8 52202 15 Oct 2001
9 52324 19 Feb 2002
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