IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
For long integrations (100 sec frame time), attitude control is returned to the observer after 80 seconds to
halt the drift.
In addition, attitude resets were performed regularly (about every 30 minutes) to return the spacecraft
attitude to the observer attitude. The system is designed to ensure that any motion to return the spacecraft
attitude to that of the observer does not take place during an IRAC integration, to avoid smearing the
images. Throughout the first 18 months of the mission the PCS system and the corresponding parts of the
IRAC AOT were being adjusted for optimal performance. Below is a guide to the astrometric accuracy
and image quality that can be expected from a typical observation.
4.12.1 Pointing Accuracy
Slews under observer control settle to the accuracy to which the star tracker to IRAC pointing offset is
known, about 0.5”. Offsets between dither/mapping moves are accurate to 0.1” relative to the commanded
move for small moves (~ 10”), and for large moves (~ 0.5 deg) the accuracy is ~ 0.5” (though this was
improved as of Spring 2005, and should be only ~ 0.2” thereafter). An additional pointing error comes
from the gyro drift which can accumulate over the 30 minute period between attitude resets. This error is
typically ~ 2” for a “worst case" frame just before a reset.
The pointing of each frame as reported in the header keywords CRVAL1 and CRVAL2 is an average of
the observer attitude during the frame, and is typically accurate to ~ 0.5” (though it may be slightly worse
for short frames where the observer has not fully settled). Other header keywords related to pointing
include RA_RQST and DEC_RQST, the requested R.A. and Dec. of the frame, and PTGDIFF, the
difference between the requested and actual pointing. USEDBPHF should be T for all frames, if not, then
pointing transfer has failed for the frame.
The Level 2 (Post-BCD) pointing refinement module is run by default in the post-BCD pipeline to refine
the pointing to 2MASS accuracy (~ 0.15”), and will be successful if there is a sufficient number of
2MASS stars in the data. The module operates by matching common stars between frames (“relative
refinement") and a fiducial set of stars from 2MASS (“absolute refinement"). The (R.A., Dec.) position
and twist of each frame is then adjusted until a global minimum in the residuals is found. Application of
this to the Extragalactic IRAC First Look Survey (FLS) data results in a mean position error for high
signal-to-noise stars with respect to 2MASS positions of 0.25”.
The pointing refinement module writes several new keywords to the header. RFNDFLAG is true if
pointing refinement was run and produced a refined solution. The refined position is given by keywords
RARFND and DECRFND, and rotation by CT2RFND. A new version of the CD matrix, given by
keywords CD11RFND, etc., is also written to reflect the new rotation angle (note that the pixel scale and
distortion are not changed by pointing refinement). If pointing refinement fails, then the header keyword
RFNDFLAG will be false and RARFND, DECRFND and CT2RFND will be set to CRVAL1, CRVAL2
and CROTA2, respectively. Note that the refined solution may be poor if the number of astrometry stars
in the frame, NASTROM, is low (i.e., 0, or only a few stars). The refined pointing keywords are used by
the post-BCD software if USE_REFINED_POINTING = 1 in the namelists. To use the refined pointing
Calibration 65 Pointing Performance