Parallel 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 Web page 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 will execute at a known (absolute) orientation,
APT will display the exact field of view for any instrument executed in parallel. If the primary exposure will execute 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. A single ACS channel may be used for a coordinated parallel observation, with, and only with, another instrument. Unlike NICMOS, coordinated parallels cannot be used to operate any of the ACS channels simultaneously. ACS exposures may not be used as both the prime and parallel exposures within the same parallel container.
In order to protect the ACS SBC detector from inadvertent over- illumination, the ACS/SBC configuration is no longer supported as a coordinated parallel.
Users frequently wish to employ ACS and WFC3 in parallel. Because users also generally want to dither their observations, exposures of these two cameras are generally taken synchronously. However, both ACS and WFC3 images must first be loaded into and then transferred from the camera buffers to the solid state data recorders (SSDRs). The ACS buffer can only hold one image, while the WFC3 buffer can hold two optical images and two or more NIR images, depending on the number of NIR readouts per image. However images from these cameras cannot be transferred to the SSDRs simultaneously, and each transfer takes about 350 seconds. It is possible to take at least three images per orbit in each of the two cameras.
In ACS pure parallel observations, an observation is taken with ACS on an essentially random area of the sky while another instrument is making prime observations. No SBC pure parallels will be allowed due to bright object concerns.
Unlike the previous two types of parallel programs, pure parallels contain only parallel visits. Use of the GO/PAR proposal category will make any visit in the program a pure parallel.
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. A list of all pure parallel datasets in the
HST archive is at:
Observers can request ACS pure parallels; however, there are many constraints which 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.
