IRAC Instrument Handbook - IRSA - California Institute of Technology

S8.9
Pipeline History Log 147
IRAC Instrument Handbook
for an extremely bright source is that the entire image appears to have a different DC level from
the preceding and following images.
1. Some AORs have been affected by long term residual images from previous observations.
For the most part, observers have sufficiently dithered so that the impact is minimal, on
processed and co-added data.
2. Note that the noise in the images and the sensitivity to point sources are not equal to our
pre-launch predictions (e.g., as available from our website until December 19, or in the
Observer's Manual versions before 4.0), although they are close. New sensitivity numbers
are available in the revised Observer's Manual (version 4.0), which was available at our
website starting ~December 19, 2003. For reference, the ratio of the new point source
detection threshold to the pre-launch advertised value, for low background observations in
30 sec frames, is 0.69, 0.75, 1.60, and 1.31 in channels 1, 2, 3, and 4, respectively. The
apparent modest decrease in sensitivity in channels 3 and 4 is under investigation.
3. Persistent images in channel 1. When a bright source (K=13 mag or brighter) is stared at
for a long time, for example, during a downlink, it will leave a persistent image in channel
1 that decays very slowly (persists for several hours or more). A persistent image
mitigation strategy involving annealing the array after downlinks has been put in place for
nominal operations. These anneals will erase the persistent images from the array, but do
not protect against persistent images from bright object observations that can accumulate
on the array before the next downlink. Science impact: left unmitigated, you will have
extra, spurious sources in your image. 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.