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ACS Insturment Handbook for Cycle 24 |
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7.9.1 Parallel ObservingParallel observing allows HST to operate several other instruments while the prime instrument is executing its observations. While the primary instrument observes a fixed target at user-specified coordinates, the parallel instrument observes at coordinates 5 to 10 arcminutes away, depending on the parallel instrument. The HST field of view following SM4 (Figure 3.1) shows the general locations of the instrument apertures adjacent to one another on the sky. Accurate relative positions for all instruments can be found on STScI's Observatory webpage in the Pointing section.The recommended method of determining the field of view for any instrument is to use APT. A Digital Sky Survey (or user supplied) image of the primary target area is displayed with an HST field of view overlay. Any desired coordinate and ORIENT combination for the primary target will then display the possible pointings of any instrument operated in parallel. If the primary exposure is to be executed at a specific (absolute) orientation, APT will display the exact field of view for any instrument executed in parallel. If the primary exposure is to be executed at a random (nominal) orient or range of orient values, APT allows the HST field of view to be rotated interactively about the primary pointing. APT can be a valuable resource for parallel observing programs, especially those designed for, or restricted to specific pointings for the parallel FOV.Certain operating limits are in place to restrict use of configurations, modes, parameters, elements, and requirements allowed for each instrument while used in parallel. Details on these limits are documented in the Call for Proposals and Primer. General information on ACS specific parallel operations are documented in the following sections for each of the three types of ACS parallel observing: coordinated, auto, and pure.Coordinated parallel observations are specified in the same Phase II observing program as the primary observations via the prime and parallel group containers in APT. Only a single ACS channel can be used for a coordinated-parallel observation, with, and only with, another instrument. Coordinated parallels cannot be used to operate any of the ACS channels simultaneously. ACS exposures cannot be used as both the prime and parallel exposures within the same parallel container.The ACS default (archival) pure-parallel program continued to execute for the community until midway through Cycle 13 when all of the “Default” HST archival pure parallel programs were discontinued to prolong the lifetime of transmitters on HST. This non-proprietary data came from programs 9575, 9584, and 9701.Archival pure-parallel programs can be queried through MAST using their Program ID. Observers can request ACS pure parallels; however, there are many constraints that can render pure parallels unselectable in any given orbit. Pure parallels will always be given lower priority than primaries, and are thus scheduled only on a non-interference basis. Users should consult the Call for Proposals for more information about pure parallel opportunities with ACS.