7.9 Parallel Observations

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 the HST FoV Geometry webpage.

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 HST Proposal Opportunities and Science Policies pages. General information on ACS-specific parallel operations are documented in the following sections for each of the types of ACS parallel observing: coordinated and pure.

ACS Coordinated Parallels

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 the WFC and SBC simultaneously. ACS exposures cannot 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 hold only one full frame image, while the WFC3 buffer can hold two full frame 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 three or more images per orbit in each of the two cameras. For observations involving WFC3 in parallel, we recommend users carefully read ACS ISR 2023-04 for the latest recommendations on optimal dither patterns.

Now that HST is operating in Reduced Gyro Mode, flexibility to schedule coordinated parallels is significantly reduced. it is no longer possible to efficiently conduct ACS and WFC3 prime-parallel observations with 180° rolls after six months to observe pairs of fields.

ACS Pure Parallels

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 them 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 HST Proposal Opportunities and Science Policies pages for more information about pure parallel opportunities with ACS.