IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
with other software, copy the non-standard keywords to their FITS standard equivalents, e.g., RARFND
to CRVAL1, CD11RFND to CD1_1 etc. Pointing refinement works well on most channel 1 and 2 data,
though short frames in fields near the Galactic poles in channels 3 and 4 will frequently have too few stars
for a good solution.
All data have a "superboresight" correction applied. Users wishing to make use of the superboresight
solutions need to set USE_REFINED_POINTING = 0 in MOPEX, as the superboresight pointing is
contained in the standard CRVAL1, CRVAL2 and CD matrix keywords (this is the recommended
pointing to be used when making mosaics etc.). Data which have had this correction applied will also
have the ORIG_RA and ORIG_DEC keywords present which contain the initial (uncorrected) pointing
estimate.
4.12.2 Jitter and Drift
Jitter is typically 0.1”. It has been measured on timescales ~ 0.04 seconds to 5 minutes. In addition to high
frequency jitter, there are modulations ~ 0.1” on timescales of 200–400 seconds. These are not expected
to noticeably affect the IRAC PSF. Gyro drift occurs for the first 80 seconds of IRAC integrations, but
again this should result in only ~ 0.1” of motion. Some amount (< 0.4”) of image smearing is expected in
short frames due to settling motions. Other instances of pointing glitches occur when one of the four
reaction wheels goes through zero velocity. To reduce stiction when the speed actually hits zero, the
wheels are given a small “bump" in torque at this point, which has been seen to result in a small (~ 0.05”),
short duration (~ 10 seconds) pointing glitch. On average, only about one crossing per hour occurs, and
they are thought to mostly happen during slews, so they are not expected to affect many IRAC images.
One manifestation of settling, jitter and drift during integrations is that the pointing of HDR short,
medium (for 100 sec HDR) and long frames are slightly different (the same is also true of repeats taken in
the same position). These differences are usually ~ 0.1 arcseconds, so they should not be a problem for
most observers, but they are large enough to show up as residuals in difference images.
Calibration 66 Pointing Performance