Spitzer can operate only one science instrument at a time, so observing is divided into dedicated instrument campaigns typically lasting from 7–21 days. Once the campaign plan for a particular observing cycle (“Baseline Instrument Campaign,” or BIC) has been established, it was published on the SSC website. Minor changes in the instrument campaign plan in response to changes in the DSN schedule, on-orbit anomalies, ToOs, highly time-constrained observations, and changes to the SODB may be made. Updated BICs are posted regularly on the SSC website.
There is a significant overhead associated with changeover from one instrument to another, due to the need to perform routine calibrations and engineering activities at the time of switch-off and switch-on. There are also performance limitations in some observing modes immediately following an instrument changeover. For example, the IRAC electronics stabilize after a warm-up of ~1/2 hour after switch-on. In addition, there is a modest reduction in pointing accuracy following a change between IRS/MIPS (which share common warm electronics) and IRAC, due to thermal distortion when the heat load is shifted on the spacecraft.
In order to achieve the required mission observing efficiency and ensure good performance, instrument changeovers were made as infrequently as possible without sacrificing science needs. The best balance between efficiency and schedule flexibility was achieved by using each instrument for several (~7–21) days at a time. This generally permited long mapping projects, as well as highly time-constrained observations. An instrument campaign could be interrupted in order to perform urgent ToO observations.
The instrument campaigns typically occurred in the order IRAC-MIPS-IRS, which conserved helium and minimized the effects on pointing performance described above. Observations requesting MIPS followed immediately by IRAC were not feasible under this scheme. The length of these campaigns was determined by the relative fraction of observation requests in the approved programs, and changed from cycle to cycle.