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Caveats with FUSE DATA
There are a number of potential anomalies that an investigator should check for in FUSE data. A few of the more important ones are outlined below, with links to other documentation for more details. A full catalog of factors potentially impacting FUSE data quality are given in Chapter 7 of the FUSE Data Handbook with the causes of these effects discussed in Chapter 4 of the FUSE Instrument Handbook.
Thermal effects on-orbit caused drifting of the relative alignment of the four FUSE channels. Observations in the MDRS and HIRS apertures often lost flux due to this problem. LWRS observations were less-affected by flux loss, but could show offsets in wavelength scale between exposures. Aperture definitions are provided in sec. 2.2 of the FUSE Data Handbook. Both effects were largely mitigated by the CalFUSE processing software, but users starting from raw data should be aware of them. See the
FUSE Data Handbook, sec. 2.3 and the FUSE Instrument Handbook, sec. 4.2 for more information.
The spectral height (perpendicular to the dispersion) varied greatly as a function of position (hence wavelength) on each detector segment due to this optical effect in the spectrograph design. Hence, even though FUSE apertures are 20-30 arcsec in height, there is essentially no spatial information available along the aperture. If multiple objects are within the aperture, their spectra will blend together. See section 7.4 of the FUSE Data Handbook.
Gain Sag and "walk":
Gain sag is caused by depleting of charge, usually from a localized spot, on the FUSE detectors. This effect causes photons to be detected offset in the dispersion direction, and effect called "walk." Periodic increases in detector high voltage levels could mitigate these effects to a certain extent. See section 7.3.4 of the FUSE Data Handbook and section 4.4.2 of the FUSE Instrument Handbook.
A variety of emission lines due to the residual Earth atmosphere are present within the FUSE band pass. The strength of these lines varies between orbital day and night, look direction, and even from early in the mission (solar maximum was 2000-1) to later in the mission. Typically the "walk" problem was most noticeable near airglow lines. See section 7.1 of the FUSE Data Handbook.
Various factors could affect the local background levels on the FUSE detectors (see section 4.3.2 of the FUSE Instrument Handbook and section 7.2 of the FUSE Data Handbook, especially Figure 7.3). Scattered light can, for instance, produce a non-zero "background," even in saturated absorption lines, which may bottom out at a residual flux level of 1-2% of the continuum due to this effect. This primarily impacts SiC data where solar-scattered light is more severe.
Users should check the "trailer files" of calibrated data for indications of anomalies during processing of a particular data set. This may provide clues to any unexpected pathologies. Users should also check the 2-D images of the detector (*.gif files) for evidence of scattered light or other unwanted signal.