IRAC Instrument Handbook - IRSA - California Institute of Technology

Table 7.1: Definition of bits in the “ pmask”.
Bit Condition
0 Not set
1 Not set
2 Not set
3 Not set
4 Not set
5 Not set
6 Not set
7 Dark current highly variable
8 Response to light highly variable
9 Pixel response to light is too high (unacceptably fast saturation)
10 Pixel dark current is too excessive
11 Not set
12 Not set
13 Not set
14 Pixel response to light is too low (pixel is dead)
15 [reserved: sign bit]
7.1.1 Bad Pixels
IRAC Instrument Handbook
Included with the BCD data are three masks, a semi-static mask (the “pmask") which contains permanent
or semi-permanent bad pixels and regions, and which is the same for all BCDs in a given AOR and
channel, the "imask" which contains bad pixels specific to any one BCD, and the “rmask” which contains
outliers masked by the post-BCD pipeline. All of the bits set in the imask indicate pixels that have been
compromised in some fashion. Not all of the imask bits are set by the BCD pipeline, but some bits are
placeholders for post-BCD processing of data artifacts. The higher the order of bit set in the imask, the
more severe the effect on data quality. Mask values are set as powers of two, and summed together for
each pixel. Any pixel with a bit set in the pmask is suspect.
Several sets of pmasks have been produced. At the start of the mission, sets were produced at 3−6 month
intervals. As the bad pixel behavior has been shown to vary little with time, these intervals were extended
to 12−18 months. The masks are made from calibration data spanning three campaigns, allowing some
short-term bad pixels to anneal out, while retaining the ones persistent on timescales of weeks or more.
Pixels consistently noisy in the darks and/or flats in these three campaign sets are flagged. The regions of
amplifier glow are also flagged (with bit 10). Combined masks are also available, with bit 0 set to indicate
Data Features and Artifacts 105 Darks, Flats and Bad Pixels