IRAC Instrument Handbook - IRSA - California Institute of Technology

Figure 5.9: Transposition of an IRAC channel 1 dark image by the IMFLIPROT module.
IRAC Instrument Handbook
Currently, channels 1 & 2 are flipped about their vertical axis, which is illustrated by the equation above.
The image flip of these two channels provides an array orientation with the E axis located to the left of N
for all channels. Since this image transposition is applied after skydark subtraction and flatfielding, those
calibration files do not have such an orientation.
5.1.18 DETECT-RADHIT (cosmic ray detection)
Within this module, individual frames are analyzed for probable radiation hits (cosmic rays), and the
results appear as a flag in the imask file. This is computed by a median filtering technique. Input images
are read in, and a median filter is applied. The difference between the input image and the median-filtered
image is then computed. Pixels above a specified threshold (i.e., are “pointier" than is possible for a true
point source) are then flagged in a mask image (bit 9 of imask is set when a pixel is suspected to be hit by
a cosmic ray).
5.1.19 DNTOFLUX (flux calibration)
IRAC flux calibration is tied to a system of celestial standards measured at regular intervals during each
campaign. The IRAC IST provides the calibration server with calibration files based upon these
measurements. Because the flux calibration is determined from stellar point sources, the calibration for
extended sources is somewhat different. For details of the photometric calibration and correction factors,
see Chapter 4. The IRAC data are calibrated in units of MJy/sr in this module. This is accomplished by
multiplying the data image by a conversion factor provided by the calibration server. This conversion
factor is written to the data header as:
/ PHOTOMETRY
Pipeline Processing 85 Level 1 (BCD) Pipeline