IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
estimate derived from the spacecraft telemetry and which provides our initial pointing estimate). This
corrected pointing history file (the “superboresight" file) is then applied to the BCD at the pointing
transfer stage of the BCD pipeline. The superboresight RA and Dec estimates are recorded in the
CRVAL1 and CRVAL2 FITS keywords, and the position angle estimate is recorded in the CD matrix
keywords. The uncorrected RA and DEC are retained, but called ORIG_RA, ORIG_DEC, as is the
pointing refinement solution for each frame (as RARFND and DECRFND). Note that to use the
superboresight solution, USE REFINED POINTING = 0 should be set in the MOPEX namelists.
Superboresight was implemented as a patch to the S13 software build, thus most (but not all) data
processed with S13 or subsequent pipeline versions will have it. Users should check for the presence of
the ORIG_RA, ORIG_DEC keywords to see if it has been applied to their data. From S14 onwards, the
HDR data have the long frame RA, Dec solution copied to the short frames, as the short frame pointing
solutions are less accurate. Neither superboresight nor pointing refinement are run on the subarray data.
.
5.3.3 Mosaicking and Coaddition
Using the refined coordinates, individual IRAC BCDs from a given observation (AOR) are reconstructed
onto a larger field (mosaicking), and overlapping frames are averaged together to achieve a higher S/N.
Outlier rejection is performed on sets of overlapping pixels. Because Spitzer observations cover such a
large area, individual BCDs are remapped onto a common grid with a technique similar to “drizzle”
(Fruchter & Hook 2002, [11]). The pixel size in the mosaics produced by the pipeline is exactly 0.6
arcseconds x 0.6 arcseconds in the final data processing. The masks are used in the coaddition in such a
way that the pixels previously flagged as bad (for example, hot or dead pixels) are rejected before the
averaging process. Cosmic rays are rejected at this point via the outlier rejection algorithm. Users will
receive a single coadded image per channel, and per observation (AOR). It will be accompanied by a
coverage map and an uncertainty file per channel per exposure time.
Pipeline Processing 96 Level 2 (Post-BCD) Pipeline