IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Figure 5.12. Profiles showing the column median versus row values for identifying muxstripe. The
muxstripe is now identifiable between rows 125 and 200 (significantly lower values than the median
background).
From each of these median-subtracted arrays, a one-dimensional array of the values along each row is
then created, simply by combining the pixel values along the X-dimension. A profile that represents the
median versus row is created. For each profile, statistics arre calculated to identify any muxstripe. It will
be identified as a deviation larger than 3% of the median value for several rows. This will miss the
weakest muxstripe or a case where all readout channels have muxstripe in the same position (cluster of
very bright stars), but this will be rare. The subset of pixels that are affected by the muxstripe is identified
and this column is corrected using the difference of the median of the ‘clean’ pixels and the median of the
affected pixels. The readout channel arrays are then read back out to recreate the original image, and the
corrected image is written to the CBCD file. More information about muxstriping can be found in Section
7.2.2.
5.3 Level 2 (Post-BCD) Pipeline
Pipeline processing of IRAC data also includes more advanced processing of many individual IRAC
frames together to form more “reduced” data products. Known by the generic title of “post-BCD”
processing, this extended pipeline refines the telescope pointing, attempts to correct for residual bias
variations and produces mosaicked images. We do not attempt to improve (relative to the BCD) the point
source or extended emission flux calibration by automatically comparing to a reference source catalog.
The mosaic only includes data from a single observation or AOR.
All IRAC BCD images contain a pointing estimate based on the output of the Spitzer pointing control
system (star tracker and gyros), i.e., the boresight pointing history file. This initial pointing estimate is
accurate to about 0.5”. The post-BCD pipeline performs additional pointing refinement for all IRAC
frames. This is achieved by running the SSC point source detector on the channel 1 and 2 frames and
comparing the resultant list of point sources to the 2MASS catalog. The results are then averaged, and the
known focal plane offsets between all four channels are applied to produce a “superboresight” pointing
history file, which is then applied to the data during end-of-campaign reprocessing. This improves the
pointing accuracy of the frame to better than about 0.3”. This refined RA, Dec appears in the header as
the CRVAL1, CRVAL2 keyword values.
The pipeline SSC mosaicker produces a single image (one per band) from many input images. First, the
BCDs are corrected for overlap consistency. The parts of the images that overlap are forced to have the
same background value via addition of an offset. Then a “fiducial frame” is derived. This is the definition
for the output frame in terms of its physical size, projection, and orientation. Because IRAC has such a
large fie ld of view, projection effects are non-negligible, and the mosaicking and coadding process must
reproject the data. The fiducial frame finder seeks to minimize the amount of “blank” area in the output
mosaic by rotating the output projection such that it is aligned with the map axes. This is useful for long
Pipeline Processing 94 Level 2 (Post-BCD) Pipeline