16 Dec 2011

PCRS peak-up has been added to 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 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 maps will be made available on the Spitzer website in Spring 2012, 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 acquistion on the PCRS, offset to IRAC, and associated telescope motion settling. For existing programs the additional time required to obtain the PCRS peak-up will not be charged to original program time allocation.

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 < 12.5 mag range to an accuracy of better than 0.14" (1 sigma radial). For PCRS peak-up to succeed, the following must be true:

1. The peak-up target must be in the magnitude range 7.0 mag < V < 12.5 mag.
2. There should be no neighbors within 40 arcseconds of the target that are brighter than the magnitude of the peak-up star + 5.5.

When adding PCRS peak-up to your observation, you have the choice of peaking up on either the science target itself or an offset star. If the visual magnitude of your target is between 7 and 12.5, then we recommend you use the target itself. Otherwise you can either use the "PCRS selection" button in Spot to choose a nearby star from the PCRS Guide Star Catalog, which contains a carefully selected subset of stars from the Tycho catalog, or specify your own offset star. Keep in mind the requirements above in choosing your peak-up star (both of which are automatically met by the PCRS Guide Star Catalog).

If you are using an offset target, we recommend that you choose the closest possible star to your science target, to optimize the slew from the offset star on PCRS to the science target on IRAC. Please make sure that the relative astrometric positions of your peak-up star and your science target are as precisely known as possible, as the efficacy of the peak-up using the guide star method depends directly on the knowledge of the angular offset between the guide star and the target.

See the Spot Version 19 Release Notes for the mechanics on how to use Spot to add PCRS peak-up to an AOR.

Putting Your Science Target on the "Sweet Spot"

After peak-up, you can ensure that your target is placed in the "sweet spot" region by specifying your target as a Fixed Cluster target in Spot, with Array (Row,Col) offsets given in the table below. (MODE = readout mode)