7.7.2 Peak-up Settings

In principle, selection of a peak-up target for an extended source can be accomplished in basically the same manner as described for a point source in Chapter 6 of this Cookbook - see that chapter for details of the peak-up configuration process. If a large region is being mapped, it may not be necessary to perform a peak-up at all (if the nominal pointing accuracy of Spitzer, $1\hbox{$.\!\!^{\prime\prime}$}0$, is sufficient to place the selected slit on the target with adequate precision). However, it should be kept in mind that each movement of the telescope introduces a random, cumulative pointing error of order $0\hbox{$.\!\!^{\prime\prime}$}2$. Thus, maps composed of large numbers of individual pointings may accumulate large positional errors by the end of a mapping cycle, and might benefit from a peak-up at the start of the cycle. In any case, peak-up is performed only once per mapping cycle, at the start of each cycle.

For this example, a Moderate accuracy IRS peak-up is selected, since a relatively large number of individual pointings will be used in the layout of the spectral map (see §7.7.3). Peaking-up on an extended source should be avoided unless there is no other option, so we will not try to use the Ring Nebula itself as a peak-up target. (In addition, the flux distribution of the Ring Nebula, which lacks a central, brightest peak, would exclude it as even a suitable extended peak-up target.) Instead, we will use one of the ``GOOD for Blue'' stars returned by the ``2MASS Selection'' function. This star has a flux density of 120 mJy in the Blue peak-up passband.

A SIMBAD search on the coordinates of the 2MASS peak-up star reveals that it is HD 175291, a K type star with $V=8.3$ mag. Searching the Tycho-2 catalog, as described in §6.9.2 in the IRS Staring Mode chapter of this Cookbook^7.1, yields accurate epoch J2000.0 coordinates of RA = 18:53:15.84 and DEC = +33:28:28.1, and proper motions of $\mu_{\alpha} = -15.8 \; {\rm mas \; yr}^{-1}$ and $\mu_{\delta} = +13.8 \; {\rm mas \; yr}^{-1}$. This information can be used to update the default values in the ``Peakup Target'' specification window in the AOT entry window (after converting from ${\rm milliarcseconds \; yr}^{-1}$ to the units of ${\rm arcseconds \; yr}^{-1}$ that are used by Spot). This star has a magnitude in the range $7.0 < V < 10.0$, so, in principle, it could also be used as a PCRS peak-up target.

Examination of the DSS, 2MASS, and IRAS images of HD 175291 shows that no brighter object is located within $1^{\prime}$. Spot's ``Background'' option (in the Target window) yields a local $100 \; \mu {\rm m}$ background surface brightness of $7.1-7.8 \; {\rm MJy \; sr}^{-1}$ (including zodiacal light). The plot of the $100 \; \mu {\rm m}$ IR cirrus brightness vs. minimum peak-up source flux density in the IRS chapter of the SOM confirms that an object with a flux density of at least $\approx 5 \; {\rm mJy}$ in the Blue passband is an acceptable IRS peak-up target against this level of IR background.