IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
program is designed to mask out stray or scattered light from stars outside the array location as well as
filter ghosts from bright stars. The module will alter each imask, corresponding to each BCD file, with
pixels likely to be affected by stray light, by turning bit 3 on for those pixels that are likely to be affected
by scattered light. The program turns imask bit 2 on for those pixels likely to be affected by filter ghosts.
The module first queries the 2MASS database, producing a table of sources that are likely to produce
scattered light within the fie ld of view. Using the BCDs and the 2MASS table, including the flux of the
bright sources, possible stray light affected areas are calculated. These pixel positions are then turned on
within the imask. When the CBCDs are combined to a mosaic, the corresponding pixels in the CBCDs
will not be used to produce the mosaic, thereby “masking out” the input pixels possibly affected by
scattered light.
IRAC data users are reminded that observations that were not adequately dithered (such as the ones made
with the small-scale dither patterns) will have gaps if the stray light mask is used. In these cases, the stray
light masking program can be downloaded from the Spitzer website and run on the BCDs in an
unaggressive mode by setting a keyword. This disables the production of the larger masks for very bright
stars (which produce diffuse scattered light over a large fraction of the array), avoiding gaps in mosaics.
More information about stray light can be found in Section 7.3.1.
5.2.2 Saturation
Many of the following artifact corrections need knowledge of the offending source’s flux to work
correctly. For observations of very bright sources, the signal (and even pedestal) reads can be saturated.
Therefore, the next step in the artifact mitigation process is the saturation correction.
For a bright, strongly saturated point source, the DN will increase from some low number away from the
source to some maximum value between 35,000 and 47,000 DN, and then decrease to a small, usually
negative number, at the center. The image looks like a bright doughnut with a dark center. This inverted
“crater” peak profile indicates that a significant fraction of the light from the bright star may have been
lost due to saturation. Recovery, or flux rectification, is possible if the point-spread-function, or PSF, of
the star is known. The PSF can then be scaled in flux until the the non-saturated pixels in “wings” of the
stellar profile can be fit correctly.
There are several steps to rectify the inner region of the saturated star. First, the exact position of the
saturated star is identified using the “craters”. The program then creates a sub-image around the saturated
star, and that is resampled on a finer grid to match the 0.24 arcsecond resampled PSF. Remaining
artifacts, such as banding and muxbleed, are masked out. The PSF is then matched pixel-by-pixe l, the
PSF flux wings are scaled to the target wings, mean flux ratios are computed, and the best fit outside the
inner saturated region is determined. The “lost flux” is then calculated and the star is rectified by
replacing the inner, saturated pixels with flux determined from the PSF profile.
The IRAC PRF in channel 1 was found to be too narrow for stars, and so a “puffed up” version was
empirically derived and found statistically to be more accurate by testing it on stars with known flux.
Pipeline Processing 90 The Artifact-Corrected BCD
Pipeline