7.4 Slit Orientation and Timing Constraints

Because Spitzer is required to point within $\approx35^{\circ}$ of the perpendicular to the Sun at all times, most of the sky can be observed for only part of the calendar year. To check a target's Spitzer visibility, click on the ``Visibility/Orientation'' button in the Spot Target window (see Figure 7.1) to bring up the Visibility Windows window (see Figure 7.2, left panel).

Figure 7.2: Spitzer visibility windows for the Ring Nebula. The left panel shows the default configuration of Spot's Visibility Windows window, while the right panel shows the configuration after requesting the IRS slit orientations for 2004 Aug 15 and selecting ``Calculate Position Angle.''

Note that the slit orientations are a function of the calendar date and cannot be requested as a special maneuver. Spitzer does not perform roll maneuvers. The only way to obtain a specific slit position angle is to constrain the proposed observations to be obtained on the date(s) during which the current spacecraft position produces the desired slit orientation. While it is recognized that some scientific goals can only be met by using a specific slit orientation, proposers are encouraged to design their programs to use as few constraints on the timing of observations as possible. All requests for specific slit orientations following from Astronomical Observation Requests (AORs) with timing constraints must be thoroughly and rigorously justified in the text of the accompanying observing proposal. Because of the limits to the range of allowed pointings of Spitzer as a function of time (as discussed in the SOM), in some cases it may never be possible to achieve a desired slit orientation for a particular location in the sky.

For illustrative purposes in this example, we will constrain the IRS slit orientation for our observation of the Ring Nebula to be within $\pm10^{\circ}$ of perpendicular to the major axis of the nebula (which is indicated in Figure 7.3). This will produce a spectral map that ``slices'' the nebula perpendicularly along its major axis. The Ring Nebula's major axis is oriented at a position angle (PA) of approximately $66^{\circ}$ (measured in the usual fashion, eastward of ${\rm PA}=0^{\circ}$ at due north). Thus, the SL slit must be oriented at

$${\rm PA} = 156^{\circ}\pm 10^{\circ} = 146^{\circ}{\rm to }166^{\circ}$$

during the spectral mapping observation. Because the slit is symmetric, a position angle rotated by $180^{\circ}$ will work as well, so

$${\rm PA} = 336^{\circ} \pm 10^{\circ} = 326^{\circ}{\rm to }346^{\circ}.$$

can also be used.

Figure 7.3: 2MASS $K_{\rm s}$ band image of the Ring Nebula. The major axis of the nebula is indicated with a grey (red) line. The roughly elliptical nebula measures $59^{\prime \prime }$ across its minor axis and $89^{\prime \prime }$ across its major axis. North is at the top of the image; east is to the left.

Finding the range of dates when a desired slit orientation on a given target is available requires an iterative examination of the Spitzer visibility windows for that target. There are a total of six visibility windows reported by Spot for the Ring Nebula (see Figure 7.2). Henceforth, we refer to these as ``Window 1,'' ``Window 2,'' and so on, in order of earliest (top) to latest (bottom).

We begin by typing the approximate start date of Window 3 into the ``Observation Date'' entry box (located mid-way down the Visibility Windows window), and then clicking on ``Calculate Position Angle.'' For example, entering ``2004 Aug 15'' returns the slit orientations for the various IRS modules on that date (see Figure 7.2, right panel). Notice that the different IRS slits are not aligned. If we were using more than one IRS module, and desired to have the same slit orientation in all modules, then the observations for each module would have to be performed on different dates (keeping in mind that for some targets it may be impossible to ever attain a given slit orientation with multiple, or even any, slits).

In this particular case, the orientation of the SL slit on the selected date ( $42^{\circ}$) does not fall into either of the acceptable ranges ( $146^{\circ}-166^{\circ}$ or $326^{\circ}-346^{\circ}$). The Spitzer PA changes (decreases) by approximately 0.5-1.0 degrees per (increasing) day depending on the ecliptic latitude of the target. Thus, we estimate that the SL module PA will enter the first of our desired ranges about 56-112 days later than 2004 Aug 15. On 2004 Oct 10 (i.e., 56 days later), Spot reports a PA of $352^{\circ}$, indicating that this date is still slightly earlier than the first day on which Spitzer enters the desired PA range. Checking the days immediately after 2004 Oct 10 reveals that a PA of $346^{\circ}$ is first achieved on 2004 Oct 17 - this is the start of the first PA-constrained visibility window. A similar process performed using dates near the end of Window 3 reveals that the PA never quite reaches the other end of the desired range ( $326^{\circ}$) before the end of the visibility window on 2004 Nov 8. Repeating this process for Windows 4-6 yields the information shown in Table 7.1. Note that Windows 3, 5, and 6 do not reach the minimum acceptable PA ( $146^{\circ}$ or $326^{\circ}$) before those visibility windows for the Ring Nebula end. In order to be implemented, these timing constraints have to be recorded in Spot as part of an AOR. This cannot be accomplished until after the AOR is completely configured, and the procedure is discussed in §7.8.1.

Table 7.1: PA-constrained SL Visibility Windows

Visibility	Start Date	PA	End Date	PA
Window		(deg)		(deg)
3	2004 Oct 17	346	2004 Nov 03	333
4	2005 Apr 20	166	2005 May 14	146
5	2005 Oct 25	346	2005 Nov 11	333
6	2006 Apr 27	166	2006 May 01	163