Operational Constraints: Definitions

Spacecraft constraints:

Visibility constraints:

Due to several operational requirements such as solar avoidance and solar panel illumination, Spitzer is constrained at all times to point the telescope no closer than 82.5 degrees toward and no further than 120 degrees away from the Sun. This defines the Operational Pointing Zone (OPZ) of the Observatory. Experienced Spitzer observers should note that the OPZ was reduced by 2.5 degrees in January 2004. This change allowed us to increase the maximum slew rates used by Observatory, which gave us faster slews and therefore more time to integrate on the sky.

The shortest period that any object is visible in the OPZ is approximately 80 days per year (in two intervals of 40 days, separated by 6 months) for targets near the ecliptic. This increases to approximately 120 days per year at +/- 45 degrees ecliptic latitude, to year-round visibility very near the ecliptic poles. This region around the ecliptic poles is referred to as the Constant Viewing Zone (CVZ). For more information, see section 3.2 of the Spitzer Observer's Manual (SOM).

Roll constraints:

The roll angle of Spitzer is at all times precisely determined by the spacecraft pointing constraints. The sunshade on Spitzer must always be kept within 2 degrees of the radial direction to the sun. Because of this, the orientation of the focal plane on the sky is a function of calendar date and the ecliptic latitude of the object. This orientation changes by approximately 0.5 to 1.0 degrees per day, depending upon the ecliptic latitude of the target. The result is that if spectra of an object near the ecliptic poles were taken using the same slit separated by one week, then the orientation of the slit on the object would differ by 7 degrees. In the case of point sources this should not be a concern, but in the case of extended sources (such as nearby galaxies), observations which are not taken in sequence will sample different regions of the object.

Instrument constraints:

• Star tracker alignments must be performed every 8 hours.

• Downlinks (communication between Spitzer and Earth) occur approximately every 24 hours. The length of downlinks can vary from 30 to 60 minutes depending on the Deep Space Network (DSN) schedule.

• Inertial reference unit (IRU) calibrations must be performed every few weeks, and star tracker to PCRS calibrations are done about once every 12 hrs (no more than about 16 hrs apart), usually in conjunction with a downlink.

• IRAC Post-Cryo AORs are restricted in length to 24 hours. This limitation is automatically imposed by Spot. However, IRAC PC observations in general should not exceed ~ 12 hours due to the necessary star tracker calibrations (see above). The 24 hour limit is to allow the use of AORs for extended duration science observations, such as exoplanets, without the SSC having to convert them into Instrument Engineering Requests (IERs), as was necessary with the old 8 hour duration limit.

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