High Precision IRAC Photometry Using PCRS Peak-Up
Updated 21 September 2015
PCRS peak-up is available through the Spot observation planning tool for
post-cryogenic IRAC observations. This option, used frequently
in the cryogenic mission with IRS, provides enhanced accuracy in
positioning a target on a science instrument FOV, and can improve
greatly the photometric precision in staring mode observations.
intrapixel sensitivity variations
(see also Repeatable Pointing)
the measured flux of a point source varies by about 8% in the 3.6
μm array (Channel 1) and 4% in the 4.5 μm array (Channel 2),
depending on where the target centroid falls on a pixel. This
"pixel phase effect" is the most significant source of correlated
noise in staring mode observations. PCRS peak-up allows the
positioning (and repositioning for multiple observations) of a
target to an accuracy of about 0.1 pixel, reducing the range of the
pixel phase effect. In parallel, we are building high precision
maps of the intrapixel photometric variation in a region
approximately 1/4 pixels on a side, centered on the intrapixel gain
"sweet spots" (positions of maximum sensitivity) for the Channel 1
and Channel 2 subarray FOVs. These maps will enable the intra-pixel
gain correction of staring mode observations of point sources
without using the science observations themselves.
The latest versions of the "sweet spot" maps are now available on the
Spitzer website, along with an IDL program to use the maps to correct aperture photometry.
The PCRS peak-up mode is designed to facilitate high precision (<1000 parts per
million) relative photometry and will not generally benefit other observations. The
peak-up takes 2-5 minutes of observing overhead per AOR for the acquisition on the
PCRS, offset to IRAC, and associated telescope motion settling.
Using PCRS Peak-Up
The Pointing Calibration and Reference Sensor (PCRS) operates in the
visual part of the spectrum (505-595 nanometers), and its main
function is to calibrate and remove the optical offset between the
star trackers and the telescope. The PCRS can measure the centroids
of stars in the 7.0 mag < V < 12.1 mag range to an accuracy of
better than 0.14" (1 sigma radial). See
here for recommendations on how to design observations using PCRS peak-up.