The FUSE Archival Data Handbook - MAST - STScI

4.2.1.3.1 Corrupted Data
Occasionally, photon arrival times in raw TTAG data files are corrupted. When this happens,
some fraction of the photon events have identical, usually large TIME values, and the goodtime
intervals (GTI) contain an entry with START and STOP set to the same large value.
The longest valid exposure spans 55 ks. If an entry in the GTI table exceeds this value, the
corresponding entry in the timeline table is flagged as bad using the “photon arrival time
unknown” flag; (see Dixon et al. [2007]). Bad TIME values less than 55 ks will not be detected
by the pipeline.
Raw HIST files may also be corrupted. OPUS fills missing pixels in a HIST image with
the value 21865. The pipeline sets the WEIGHT of such pixels to zero and flags them as bad.
This is done by setting the photon’s “fill-data bit” (see Dixon et al. [2007]). Single bit-flips are
corrected on-board, but occasionally a cosmic ray will flip two adjacent bits. Such double bitflips
are not detected by the correction circuitry, producing (for high-order bits) a “hot pixel”
in the image. The pipeline searches for pixels with values greater than 8 times the average of
their neighbors, and masks out the higher-order bits.
One or more image extensions may be missing from a raw HIST file. If no extensions
are present, the keyword EXP STAT in the IDF header is set to −1. Exposures with nonzero
values of EXP STAT are processed normally by the pipeline, but are not included in the
observation-level spectral files ultimately delivered to MAST. Though the file contains no data,
the header keyword EXPTIME is not set to zero.
4.2.1.3.2 Airglow Observations
When creating the IDF files for bright-earth observations (programs M106 and S100) or 900level
airglow exposures, CalFUSE sets the header keyword EXP STAT = 2. For 900-level
exposures, the SRC TYPE is set to EE, and the following warning is written to the file header:
“Airglow exposure. Not an astrophysical target.” The pipeline then processes the file as usual.
In particular, the limb-angle flag is set in the timeline table (extension 3 of the IDF), and the
“Time with low limb angle” is written to the file header. However, the following things change:
• no jitter correction is performed;
• when the time-dependent flags are copied from the timeline table to the individual photons,
the limb-angle flag is masked out; and
• when the good-time intervals and exposure time are calculated, the limb-angle flag is
ignored.
The extracted spectra include photons obtained at low limb angles. The detector-image
plots (*spec.gif) do, too. But the count-rate plots (*ext.gif; *rat.gif) still indicate times
when the line of sight passed below the nominal limb-angle limit.
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