IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 7.6: IRAC channel 1 (left) and channel 2 (right) observations of a crowded field with column pulldown
apparent from the brightest sources. Note that the brighter sources affect a larger number of columns.
These data were taken from program pid = 613, AORKEY = 6801408.
7.2.5 Row Pull-Up
In addition to muxbleed in channels 1 and 2, there may be electronic banding, which is manifest as a
positive offset for rows that contain bright pixels. This effect is at least an order of magnitude smaller
than muxbleed. Electronic banding is more significant in channels 3 and 4 but it is not as significant as
the optical banding in those channels (see Section 7.3.2). The BCD pipeline mitigates against these
effects. The algorithm finds instances of pull-up and banding and fits the DC offsets on either side of the
triggering source to them.
7.2.6 Full-Array Pull-Up
In all four arrays, there is also an effect where an entire image is uniformly offset by some amount of
DN’s that is approximately proportional to the total flux or fluence integrated over the array. It is easily
noticed in a mosaic when overlap correction is turned off, and when the mosaic contains areas with and
without strongly saturated stars. We call this effect "full-array pull-up,” but it is also known as "droop" to
the community of users of doped silicon IBC arrays. The effect can go unnoticed when overlap correction
is done in the mosaic. It has no significant effect on aperture photometry of point sources or extended
sources when a good background mean can be obtained within the same 5 arcmin x 5 arcmin image as the
source. The effect is largest in channels 3 and 4, and if uncorrected, can lead to significant errors in the
derived flux of extended objects, and especially in the brightness of the background itself. It is hard to
Data Features and Artifacts 114 Electronic Artifacts