IRAC Instrument Handbook - IRSA - California Institute of Technology

Pipeline History Log 146
IRAC Instrument Handbook
These sources have a PSF that is wider than the actual true source PSF. Dithering helps to get rid
of these spurious sources.
4. Persistent images in channel 4. These are different in nature from the channel 1 persistent images.
A bright source leaves a persistent image that can last for more than a week and even through
IRAC power cycles. These images keep building up on the array. However, the amplitude of the
persistent images is rather low. Annealing has been found to erase also the channel 4 persistent
images. Therefore, we will anneal both channels 1 and 4 simultaneously, every 12 hours (after
each downlink), to erase persistent images. Again, dithering helps to get rid of these spurious
images.
5. Diffuse stray light: All IRAC images contain a stray light pattern, resembling a "butterfly" in
channels 1 and 2, and a "tic-tac-toe" board in channels 3 and 4. These artifacts are due to zodiacal
light scattered onto the arrays, possibly reflected from a hole in the FPA covers above the channel
1 and 2 arrays, and from reflective surfaces outside the edges of channel 3 and 4 arrays. The stray
light scales with zodiacal light, which is the light source for our flatfields, so the stray pattern
contaminates the flats. As a result, the flatfields will aesthetically remove the stray light rather
well from images but will induce systematic errors of approximately 5% in flux calibration for
point sources that fall in the peak stray light location. Dithering will mitigate this effect, because
it is unlikely that a dithered observation will keep a source within the stray light lobes. Diffuse
stray light will be removed from both the flatfields and the science frames in a future version of
the pipeline.
6. Stray light from point sources. Spot allows you to overlay stray light boxes on any image; if a
bright star is placed in those boxes during an observation, a scattered light patch will appear on
the array. We have found three more such boxes during testing, in channels 1 and 2. The new
stray light boxes are included in Spot now and are also shown in the new Observer's Manual.
Channels 3 and 4 have less stray light, and the stray light inducing regions are not the same as the
ones we guessed (by analogy to channels 1 and 2) from the lab tests, so the channel 3 and 4 boxes
were removed from Spot. In channels 3 and 4 the stray light arises when a star lands on a thin
region just outside the array (the same region that causes the "tic-tac-toe" pattern from diffuse
stray light in flat fields). A redundant observing strategy will help eliminate stray light problems.
Observers covering fields with bright sources should inspect the individual images; this is
required if the depth of coverage is less than 3, to identify spurious spots and rays that could be
mistaken for real astronomical objects.
7. Dark spots on pick-up mirror. There is contamination on the mirror, which causes a dark spot
about 10 pixels wide in channels 2 and 4. This is a 15% effect. Flatfields completely correct for
this feature in the data.
8. Muxbleed. We have a correction algorithm, but the coefficients need fine-tuning. Furthermore,
for bright sources, muxbleed does not scale linearly with source brightness, so even a
sophisticated algorithm cannot accurately remove it. Some experiments at fitting the muxbleed
for bright sources indicate that the decay pattern is always the same, and only the amplitude
appears to be variable.
9. Banding and column pulldown. A bright source on the array will cause its column to be pulled
down by a small amount. An algorithm to cosmetically correct the images for column pulldown
has been developed and is being tested. This appears to be an additive effect. An analogous effect