Spitzer Documentation & Tools
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.

Because of 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.