IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
thin maps, where potentially the output mosaic could be very large, but with a great deal of empty space.
The mosaicker then reprojects all of the input data onto the output projection. It reads the SSC WCS,
which contains the field pointing center, rotation, scale, and instrument distortion, and reprojects this onto
a standard TAN FITS projection. In the process, the data are undistorted. The reprojected images are
interpolated onto the fiducial image frame with outlier rejection, rejecting radiation hits that happen in
overlapping observations. The outlier rejection scheme is specifically designed to work well in the case of
intermediate coverage and may not be adequate for all observations and science programs. In addition to a
sky map (in units of surface brightness), a noise image and coverage map are also produced.
The post-BCD pipeline modules have been made available for general public use as part of the MOPEX
tool. They consist of a number of C-modules connected via PERL wrapper scripts. Namelists are used for
input. In most cases their operation simply consists of supplying the software with a list of input image
files; by default they read and understand the IRAC image headers.
5.3.1 Pointing Refinement
To improve the ~ 0.5” blind pointing, a pointing refinement is run in which the point sources are
identified in the IRAC frames and astrometrically correlated with stars near the source position in the
2MASS catalog. The pointing refinement typically improves the positional error to < 0.3” and removes
any systematic offsets.
First, point sources are extracted from the pipeline-processed mosaics and transformed to RA and Dec
using the transformations derived from the current pointing. If there are less than five sources in an
image, then there will be no refinement for that BCD.
A comparison is made of the position and flux of each point-source match found in the 2MASS pointsource
catalog. The new translational and rotational reference frame can be computed from the
differences and uncertainties, and a refinement is made of the celestial pointings and angles of each BCD
in the observation or AOR used for the mosaic. These refined values are written to the end of the FITS
headers as RA_RFND, DEC_RFND, including many others with RFND as an indicator of “refined"
pointing.
5.3.2 Superboresight Pointing Refinement
Pointing refinement operates on each IRAC channel independently. This often results in poor pointing
solutions for channels 3 and 4, in which stellar fluxes are lower and the background higher than in
channels 1 and 2. We have therefore developed a technique which combines the results of pointing
refinement in channels 1 and 2 and applies it to all four channels using the known offsets between the
IRAC fields of view. This improved pointing solution is derived during campaign reprocessing. The
results of the pointing refinement from the first run of the post-BCD pipelines are averaged, and the
correction derived from this is applied to the boresight pointing history file (which contains the pointing
Pipeline Processing 95 Level 2 (Post-BCD) Pipeline