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 SM3B ( 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 the Visual Target Tuner
(VTT). 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, the VTT
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, the VTT
allows the HST field of view to be rotated interactively about the primary pointing. The VTT
can be an invaluable 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 operate ACS channels simultaneously, the use of ACS auto-parallels is described in the following section.
In order to protect the ACS SBC detector from inadvertent over illumination, the ACS/SBC configuration may be used as a coordinated parallel only if an exact spacecraft orientation (ORIENT
ation) is specified, the coordinates of the parallel field are determined, and the parallel target or field passes the same bright-object screening applied to SBC primary observations. The Visual Target Tuner
will greatly assist in defining this type of ACS parallel program.
Currently ACS and WFPC2, used together in a coordinated parallel program, result in the ACS auto-parallel capability being disabled. This restriction is made to prevent potential conflicts between ACS buffer dumps and WFPC2 readouts. The user may attempt to restore autoparallels via PAREXP=MULTIPLE.
The ACS auto-parallel capability is intended to increase the scientific return of the instrument by adding exposures with the parallel detector while interfering as little as possible with the observer's primary program. When either the WFC or HRC is the primary channel, and the exposure in that channel meets the requirements stated below, an auto-parallel observation will be automatically scheduled in the parallel channel during Phase II processing. Parallel detector exposures will be added automatically with the longest possible exposure time that does not interfere with the primary program. In order for an auto-parallel to be scheduled, the primary observation must meet the specifications that depend primarily on the exposure time and the filter selection of the primary exposure.
The user has three control options: This is done by selecting the PAREXP
optional parameter. A user may either choose to explicitly add the auto-parallels by choosing PAREXP=MULTIPLE; choose to have no auto-parallels added by selecting PAREXP=NONE;
or leave the special requirement set to the default, DEF
. When DEF
is selected, auto-parallels will be added according to the same primary exposure requirements as those for the MULTIPLE option. When MULTIPLE is selected an auto-parallel will be added for each CR-SPLIT part of the primary exposure (see Figure 7.9). There is a simple algorithm that the Phase II software follows in order to determine if an auto-parallel is feasible:
DUR, MAX
DUR,
or RT
ANALYSIS special requirements.
Primary exposure type |
Multiple auto-parallel scenario (n ![]() |
---|---|
HRC |
n × 495 |
WFC compressed |
n × 406 |
WFC uncompressed |
n × 465 |
There are a few things to remember about auto-parallels:
The following policies will be in effect for Auto-Parallel observations:
GAIN
setting will be 2 (see Section 4.3). For HRC parallels added to prime WFC exposures the GAIN
will be 2.
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This data loss will be considered routine and it will be the policy of STScI not to repeat observations scheduled as auto-parallels which exhibit partial data loss due to the data compression. |
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 currently comes from programs 9575, 9584, and 9701. A list of all pure parallel datasets in the HST archive is updated regularly at:
http://www.stsci.edu/instruments/parallels/retrieve.html
.
Pure parallel observations are executed at every possible opportunity, although 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.
ACS auto-parallels can be added to ACS pure and ACS coordinated parallels by default if scheduling constraints allow. However auto-parallels cannot be added to observations that make use of EXPAND or MAX DUR special requirements. Therefore ACS pure parallels can either be crafted to expose for the maximum duration allowed in each individual orbit by using EXPAND/MAX DUR or have auto-parallels added, but not both.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |