Spitzer could operate only one science instrument at a time, so observing was divided into dedicated instrument campaigns typically lasting from 7–21 days. Once the campaign plan for a particular observing cycle (see section 3.1) (“Baseline Instrument Campaign,” or BIC) had 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, highly time-constrained observations, and changes to the SODB may have been made. Updated BICs were posted regularly on the SSC website.
There was 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 were also performance limitations in some observing modes immediately following an instrument changeover. For example, the IRAC electronics stabilized after a warm-up of ~1/2 hour after switch-on. In addition, there was 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 permitted long mapping projects, as well as highly time-constrained observations. An instrument campaign could be interrupted in order to perform urgent ToO observations, with the proviso that it had to use the instrument then in use.
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.
During the warm mission only the IRAC instrument was operating. It was cycled on and off only twice between May 2009 and January 2020, both times due to anomalies.