The PCRS peak-up option (see also Section 4.12.2) was designed to facilitate high precision (<1000 parts per million) relative photometry, and it introduced an overhead of 2 – 5 minutes into the observation. The PCRS peak-up option used the spacecraft's optical pointing system for peak-up. This option provided enhanced accuracy (down to about 0.1 pixels) in positioning a target on a science instrument default pixel (full array or subarray), and improved greatly the photometric precision in staring mode observations (e.g., exoplanet observations) by reducing the range of the pixel phase effect (see Section 4.6). The pixel phase effect is very well characterized and minimal in a sweet spot region, approximately 1⁄4 pixels on a side, near the center of the subarray in channels 1 and 2.
The PCRS was able to measure the centroids of stars to an accuracy of better than 0.14 arcseconds (1σ radial). For the PCRS peak-up to succeed, the peak-up target had to be in the Vega magnitude range 7.0 mag < V < 12.5 mag, and there should not have been any stars brighter than V =13.0 Vega mag within 40 arcseconds of the peak-up target. Peaking up could be done on either the science target itself or on an offset star. It was possible to select stars from the PCRS Guide Star Catalog, which contains a carefully selected subset of stars from the Tycho catalog, or specify your own offset star.
One important scientific benefit of the PCRS peak-up was the ability to reposition a target onto the same spot on a pixel during a long stare, thus enhancing the precision of time-domain measurements. Typically, the Spitzer pointing system drifted systematically by approximately 0.35 arcseconds per day (this is in addition to any potential short-term drift due to a change in the pitch angle; see Section 4.12.4 for more information). This meant that in about 12 hours, a target placed halfway between the center and the edge of the well-characterized 0.25-pixel radius sweet spot of the channel 1 or channel 2 subarray, had a good chance of drifting out of the region of minimal pixel response variation.
Although not a separate mode, exoplanet observations and some other observations were often taken in a so-called staring mode, combined with the PCRS peak-up. In this mode, the repeat option was used instead of dithering, resulting in hours of uninterrupted observation. The observers were encouraged to add a separate 30-minute AOR at the beginning of their observations, pointed off the target itself, to account for an initial drift in telescope pointing that usually stabilized after 30 minutes, and also a short 10-minute AOR, pointed off the target, at the end of the observation, to help form an additional skydark to be subtracted from their long-term staring observations.
3.3 Map Grid
If the mapping mode was used, a rectangular map grid needed to be specified in either array or celestial coordinates. First, in array coordinates, the map grid was aligned with the edges of the array, such that the map rows and columns corresponded to rows and columns of the array. Specifically, at the ecliptic, a column was along a line of constant solar elongation, and a row was along an ecliptic parallel (line of constant ecliptic latitude). It is worth noting that due to the instrument configuration, the two IRAC fields of view were at approximately constant solar elongation, so that a map with 1 column and several rows made a strip along the direction of the separation between the two fields of view, and yielded 4-array (2-array in the warm mission) coverage along part of the strip (if it was long enough). And, second, if the mapping was done in celestial coordinates, the rows and columns corresponded to J2000 right ascension (R.A.) and declination (Dec.). A position angle, degrees E of N, could be specified to orient the raster in equatorial coordinates. Specifically, if the position angle was zero, a column was along a line of constant R.A., and a row was along a parallel (a line of constant Dec.). The map could be offset from the specified coordinates by giving a map center offset.
If “No mapping” was selected in the AOT, the observation consisted of a single position at the coordinates specified in the Target section of the AOT. With “No mapping” selected, and selecting both fields of view, first the 4.8/8.0 μm field of view was pointed at the target, and then the telescope repositioned so that the 3.6/5.8 μm field of view pointed at the target. In both cases, data from all 4 (2) arrays were collected during the cryogenic (warm) mission, whether they were pointed at the target or not.