IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
It is also at this stage that “derived" parameters, most notably the integration time, are added to the
headers. The integration time is related to the Fowler number (AFOWLNUM) and the number of wait
periods (AWAITPER) via
EXPTIME = mode ∗ (AWAITPER + AFOWLNUM) (5.1)
The integration time is stored in the header in the keyword EXPTIME. Another timescale of importance
is the frame time. This is the actual length of time that the observation was integrating on the sky, and is
equal to
FRAMTIME = mode ∗ (AWAITPER + 2 ∗ AFOWLNUM) (5.2)
The factor “mode” is equal to 0.2 seconds for full-array mode, and 0.01 seconds for subarray mode. The
read-mode is determined by the least significant bit of the ancillary keyword AREADMOD. If
AREADMOD is 0 (or even) then the mode is full-array. If it is 1 (or odd) then the image is sub-array.
Note that because of TRANHEAD processing, the headers of the raw data and the final BCD data
products are not identical. In general, users should only need to read the BCD headers. However, if it
becomes somehow necessary to examine any of the camera telemetry (voltages, currents, etc.), then they
can be read from the complete raw data header.
5.1.4 INSBPOSDOM (InSb array sign flipping)
The IRAC InSb arrays (channels 1 and 2) were operated in such a way that flux appears "negative" in the
raw data (Figure 5.2). That is, data numbers start at 65,535 (16-bit max) for zero light levels and become
increasingly close to 0 as light levels increase. The INSBPOSDOM module rectifies this so that
increasing DN yields increasing flux (0 to 65,535), as is more common. This is done by
Aout = (65,535 − Ain)
where A is the pixel intensity in DN for the two InSb arrays (ACHANID = 1 or 2). ACHANID is turned
into CHNLNUM in the BCD header by the last step in the pipeline.
Pipeline Processing 71 Level 1 (BCD) Pipeline
(5.3)