5.6 Constraining the Observations

Due to positioning constraints on the observatory, Spitzer does not have the freedom to view any piece of the sky at any time at any position angle. This is because of hard pointing constraints of the spacecraft, required to keep the solar shield positioned to protect the telescope and instruments, as well as to provide power through the solar panels. Therefore, the orientation of the Spitzer detector arrays will depend upon when the observations are scheduled. It is worth noting as well that some position angle (PA) selections may never be possible to achieve (for example, for objects in the ecliptic, Spitzer has an extremely narrow range of permissible PAs).

We have developed in this chapter, using MIPS, and in the previous chapter, using IRAC, imaging campaigns to image the central region in five galaxy clusters. It is worth emphasizing again that the PA of the MIPS and IRAC observations will depend upon when these AORs are scheduled for observations. For this example, it doesn't really matter what PA we use, but it would be preferable that both the IRAC and MIPS observations are taken with the same (or at least similar) position angle. We accomplish this by imposing constraints on the observations as follows:

In the main Spot window, under the ``Tools'' pulldown menu, select ``Group/Follow-on Constraints.'' This brings a constraint editor window, as shown in Figure 5.10.

**Figure 5.10:** The constraint editor entry dialog.
$\begin{figure}\centering \epsfig{figure=figs4f/spot_constraint_editor.ps, width=5.5in} \end{figure}$

We select ``Add Group Within'' constraint, and the ``Group Within Parameters'' pop-up window will appear. We will fill the parameters in a moment. But first, what is a ``group-within'' constraint?

A group within constraint means that the AORs constrained will all be executed within a selected period of time, pre-determined by the observer. Before entering any numbers, we have to be careful here, because of the way Spitzer functions.

At any given time, only one instrument will be on, and will operate in ``campaigns'' of duration 7-14 days, before a switch is made to one of the other instruments. The switch-over itself takes some time, for the telescope to be adjusted to the required temperature, instrument calibrations to be performed, and so on. All of this is to say that selecting a very short time-scale for a group within parameter will mean that your observations may very well become unschedulable. Of course, the flipside is that using a very long time for the group of observations means that the PA could vary significantly within the group.

A generic rule-of-thumb is that the default PA changes by $\simeq 1^\circ/{\rm day}$ at high ecliptic latitude, and $\simeq 0.5^\circ$ per day for mid-latitude regions. For this example, we select 14 days for the group within parameters; this will mean that the PA will vary by no more than $14^\circ$ for the MIPS/IRAC observations, which seems acceptable, and yet the observations are not so tightly constrained to pose scheduling difficulties. A screen capture of the completed ``Group Within Parameters'' pop-up window is shown in Figure 5.11.

After clicking ``OK'' we are returned to the main constraint editor window. Now all we need to do is specify which AORs will be grouped within this constraint. We will do this by dragging and dropping the AORs we have already created.

**Figure 5.11:** The group within parameter entry dialog.
$\begin{figure}\centering \epsfig{figure=figs4f/deep_imaging_mips_group_within_parameters.ps, width=5.5in} \end{figure}$

From the main Spot window, click and hold the left mouse button on the ``IRAC mapping: a2218 center position'' AOR. Then drag the AOR into the constraint editor window and drop in the constraints sub-window. Repeat this for the ``MIPS Photo a2218 24+160'' and ``MIPS Photo a2218 70 map'' AORs. A screen capture of the resulting constraint editor window is shown in Figure 5.12.

**Figure 5.12:** The completed constraint editor dialog, with A2218 IRAC and MIPS observations constrained with a group-within constraint of 14 days.
$\begin{figure}\centering \epsfig{figure=figs4f/deep_imaging_mips_a2218_constraint.ps, width=5.5in} \end{figure}$

Clicking ``OK'' returns us to the main Spot window, and we see that the boxes under the ``G'' column are checked for the three AORs, indicating that a group constraint has been applied.

That's it. Save your AORs and targets. Then do some copying and editing (within Spot) to develop the grouped AORs for the other targets. A screen capture of the constraint editor window with all of the targets entered is shown in Figure 5.13.

**Figure 5.13:** The completed constraint editor dialog, with all targets having their IRAC and MIPS observations constrained with a group-within constraint of 14 days.
$\begin{figure}\centering \epsfig{figure=figs4f/deep_imaging_mips_all_constraint.ps, width=5.0in} \end{figure}$

The main Spot window after the complete, constrained IRAC and MIPS campaign has been designed is shown in Figure 5.14.

**Figure 5.14:** The Spot menu after the IRAC and MIPS AOT parameters have been entered for all of the targets, and the group within constraints imposed.
$\begin{figure}\centering \epsfig{figure=figs4f/deep_imaging_aot_alldone.ps, width=5.5in} \end{figure}$

That's the entire program! Don't forget to save both the target and AOR list. For reference, the raw text .aor file for this set of observations is shown in Appendix B. For reference, the raw text .aor file for this set of observations is available at the SSC website:

The total program time to execute this series of AORs is 22.2 hours. For CP-4, this would be classified as a ``small program'' $(\leq 50 \; {\rm hrs})$ . May the TAC be with you...