6.9.2.2 Option 2: PCRS Peak-up

A PCRS peak-up utilizes Spitzer's Pointing Calibration and Reference Sensor. The PCRS is sensitive at 5050-5950 Å, and can find the centroid of a star with $V = 7.0-10.0$ mag^6.2. As with an IRS peak-up, either the science target or an offset star can be used as the peak-up target. However, because the PCRS obtains images in a very different wavelength region from that of the IRS, it is strongly recommended that only an object with a known visual magnitude be used as a PCRS peak-up target. At $V \approx 12.9$ mag, 3C 273 is itself too faint to act as a PCRS peak-up target. Spot v14.2 (and later versions) has the capability to search the PCRS catalog and identify appropriate stars. The accuracy of target placement in an IRS slit for a PCRS peak-up is comparable to a High accuracy IRS peak-up ( $0\hbox{$.\!\!^{\prime\prime}$}4$). As described previously for selecting an IRS peak-up target, the PCRS peak-up target should be the brightest object in the $40^{\prime\prime} \times 40^{\prime\prime}$ field-of-view of the PCRS array. Also, because the PCRS pixels are quite large ( $10^{\prime\prime}$), it is important that the peak-up target does not have a close, bright companion. In general, in order to minimize the detrimental effect on the measurement of peak-up target position, the angular separation between a peak-up target and companion star must be larger for brighter companion stars. A quantitative expression of this condition exists, but cannot be provided here^6.3. To search for a PCRS peak-up star, the observer should select "PCRS Peak-up" and then "PCRS selection". This will bring up a window with a list of PCRS stars. Proposers should keep in mind that there is no advantage to choosing a PCRS peak-up star that is at the bright end of the allowed range over choosing one at the faint end. Then by just selecting the appropriate star and clicking on OK, the coordinates, epoch and proper motion are passed on to Spot. For 3C273, the PCRS-GSC contains one suitable PCRS peak-up target within $30^{\prime}$ of 3C 273: which has a magnitude of $V = 9.27$ and is located $24\hbox{$.\!\!^{\prime}$}73$ away at epoch J2000.0 coordinates of RA = 12:27:42.31 and DEC = +01:50:14.7. A SIMBAD search around the coordinates of this star confirms that, by coincidence, it is HD 108473, the same star identified earlier as a suitable IRS peak-up target. Figure 6.10 shows how the finished ``Peak-Up Settings'' section of the IRS Staring Mode AOT entry window would look if configured for a PCRS peak-up on the star selected from the PCRS-GSC.

Figure 6.10: Completed ``Peak-Up Settings'' portion of the IRS Staring Mode AOT for a PCRS peak-up using a star selected from the PCRS-GSC.

The PCRS-GSC contains $\sim 2 \times 10^{6}$ stars and was, therefore, necessarily constructed via an automated process that relied primarily on star catalogs. Although the best effort has been made to ensure the validity of the objects in the PCRS-GSC, it is estimated that $\lesssim 5\%$ might be questionable. Future versions of the catalog will remove these as they are discovered; in the meantime, it is advisable to check for nearby neighbors and/or overlapping (extended) background objects. This can be accomplished using the process described above for searching for bright, nearby neighbors to IRS peak-up stars selected from the 2MASS-PSC. If you find a star in the PCRS-GSC that is not a valid peak-up target, please notify the Spitzer Helpdesk, at

help@spitzer.caltech.edu.