6.5 Checking Slit Orientation

Because Spitzer is constrained 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. All locations in the sky will be observable by Spitzer for at least $\sim 80$ days per year even if Bright Object Avoidance is enabled. Target visibilities range from $\sim 80$ days per year for targets in the ecliptic plane, to $\sim 120$ days per year at $\pm45^\circ$ ecliptic latitude, to year-round visibility very near the ecliptic poles. To check a target's Spitzer visibility, click on the ``Visibility/Orientation'' button in the Spot Target window (see Figure 6.1) to bring up the Visibility Windows window (see Figure 6.2, left panel).

Figure 6.2: Spitzer Space Telescope visibility windows for 3C 273, showing the default configuration of Spot's Visibility Windows window (left) and the configuration after requesting the IRS slit orientations for 2005 July 04 (right).

If, for scientific purposes, a specific slit orientation is required, then an iterative process must be performed to determine the dates within the Spitzer visibility windows when that orientation is available. Start by typing a date that lies within one of the visibility windows into the ``Observation Date'' entry box (located mid-way down the Visibility Windows window) and click on ``Calculate Position Angle.'' For example, entering ``2005 Jul 04'' returns the IRS slit orientations on that date (see Figure 6.2, right panel). Additional dates can be checked in the same fashion to determine the range of dates during which the desired orientation is achieved. 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. 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. While it is recognized that some scientific goals can only be accomplished by using specific slit orientations, proposers are encouraged to design their programs to use as few constraints on the timing of observations as possible. Constraining observations to occur with a certain position angle requires adding a timing constraint to the AOR - this procedure is discussed in the IRS Spectral Mapping chapter of this Cookbook (see Ch. 7).