IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
these persistent images has been identified as a known feature of the flight array (broken clamp) that
cannot be fixed. The longer-term persistent images in channel 4 are induced by bright mid-infrared
sources or bright stars. The channel 4 persistent images have very unusual properties: they have lasted for
as long as two weeks, they can survive instrument power cycles, they do not decay gradually, and they
can switch sign, as they decay, from positive to negative. The amplitude and decay rate of long-term
persistent images is variable and no secure model exists to remove these artifacts from the data.
Figure 7.8: Median of channel 1 i mages from a calibrati on observation performed after observing Pol aris.
The 5 bright s pots are persistent images from staring at the star while observing, while the set of crisscrossing
lines were generated by slews between the various pointings. These observations were taken from
AORKEY=3835904, in program pid=19.
We instituted a proactive and highly successful method of eliminating persistent images. Channels 1 and
4 were temporarily heated, or “annealed," briefly, with a small current running through the detector. The
arrays were annealed after every telemetry downlink, which erased any persistent images built up during
the downlink or during the previous 12-hour period of autonomous operations (PAO). This strategy,
combined with scheduling known bright object observations immediately before downlinks, greatly
decreased the possibility that preceding observations produced persistent images.
We have found that stars brighter than about magnitude −1 at 3.6 microns, when observed for more than
about 6 seconds, leave a residual image that persists through an anneal, and even through multiple
Data Features and Artifacts 117 Electronic Artifacts