Spitzer Space Telescope

Post-Launch updates to IRAC - 19 Dec 03

Dear Spitzer/IRAC Observers,

If you are receiving IRAC data from the first campaign, or considering a
General Observer proposal, you should be aware of the following issues
with the instrument that affect the quality of IRAC data. Some of these
issues will be eventually corrected or eliminated, but they are still
present in the data that are currently released.

First, 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 will be
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.

Second, please note that many of the instrumental artifacts known before
launch and discovered in flight are avoided or seriously reduced in
observations that are well dithered. The SSC continues to recommend taking
a minimum of three images of your science target as a best practice for
observing with IRAC.

Below is a list of issues that were uncovered in flight.

1.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.

2.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.

3.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 flat fields, so
   the stray pattern contaminates the flats. As a result, the flat fields
   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 flat fields and the science
   frames in a future version of the pipeline.

4.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.

5.Dark spots on pick-up mirror. There is contamination on the mirror which
   causes a dark spot about 10 pixels wide in channels 2 & 4. This is a
   15% effect. Flat fields completely correct for this feature in the
   data.

6.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.

7.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 for an extremely bright source is that the entire
   image appears to have a different DC level from the preceding and
   following images. The physical origin of these effects and the probably
   related (and already known) banding effect is not yet understood. This
   work is in progress.