IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
with no light incident on the array. The signal from the darkest background in a 100 second frame in
channel 3 is only about 1000 electrons. Channel 1 has much smaller offsets, but the sky is so dark that the
offsets are often larger than the background signal. Channel 2 has very small offsets, which are less than
the background signal except in short frames or certain frames immediately following a change in
integration time. The background in channel 4 is so large that the offsets are almost negligible except in
very short integrations. There is no measureable excess noise from the offset itself: the noise is not the
square root of the equivalent number of charge quanta on the integrating node. This is because the offset
arises from the redistribution of charge within the mux in which the associated currents and capacitance
are much greater than in the detector diodes. However, imperfect correction of this “first-frame” effect
does increase the uncertainties in BCD frames. The uncertainty scales with the size of the offset and its
small-scale spatial nonuniformity. Only in channel 3 does it significantly increase the total pixel noise.
We can break down the offset into contributions beginning with the largest spatial scale down to the
smallest. In this view, the largest part of the offset is uniform over the array, followed by the contribution
of a few spatial gradients, and some pinstriping that repeats every four columns (due to the four array
outputs), with a few columns with odd offsets (due to hot pixels or parts of the mux), and weakest of all,
pixel-to-pixel dependent offsets.
There are some very obvious features imposed on the true offset, due to a relatively small number of hot
pixels, and mux glow. Hot pixels usually appear bright, and in such cases one can see a trail of muxbleed
(in raw images) or a pinstripe pattern in InSb arrays, or the bandwidth effect (in Si:As arrays) following
the hot pixel. These pixels have high dark currents and are usually isolated, but sometimes in a clump.
“Dead” pixels are really just very hot pixels, so hot that they saturate before the first pedestal sample. In a
BCD image, hot pixels do not appear bright because they have been canceled by the labdark or skydark
subtraction. Most hot pixels appeared after launch and are the result of hits by energetic nuclei. By
annealing the arrays, we restored most pixels that got activated. Some of them cannot be restored, and
thus they became “permanent” hot pixels. Some pixels jumped randomly from normal to high dark
current and back, dwelling in one state for anywhere from a few minutes to weeks, so they may not be
canceled by a skydark subtraction. These are IRAC's “rogue pixels.” The IRAC “static” bad pixel masks
were updated when significant changes in the permanent bad and/or hot pixels occurred.
Areas of mux glow are visible in the labdark and images. Electrons and holes recombine in diodes in the
mux, allowing current to flow. Photons emitted in the recombination are detected in the InSb or Si:As
detector above or near the source of the glow. Most prominent is the glow from the four output FETs
visible only in Channels 3 and 4 (the Si:As arrays). These are semicircular areas about 17 pixels in radius
located near column 256, row 30 at the right edge of the images. The glow is most obvious in long
frames. Another glow region is visible along the last few rows in all 4 channels; it comes from the unit
cell FETs. Currents flow through all the unit cell FETs in the last row which is left selected during the
integration, so the glow is particularly bright in the last row itself. The 3rd and faintest glow region is
along the left edge (column 1) of channel 3. Detected glows have shot noise, which can exceed the
background noise along the last row and in the brightest parts of the semicircular areas. Pixels are masked
in these areas where the noise significantly degrades sensitivity in 100-second frames.
Data Features and Artifacts 104 Darks, Flats and Bad Pixels