Space Telescope Science Institute   7.8.1 Determining Orientation for Phase II  7.10 Pointing Stability for Moving Targets

7.9 Parallel Observations


7.9.1 Parallel Observing


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.

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. 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 (ORIENTation) 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.

ACS Auto-Parallels

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:

  1. The primary exposure must be for an external target, in ACCUM mode, and either in the WFC or HRC channel. It is not possible to observe simultaneously with the HRC and SBC since they share the same optical train up to the flip mirror. Simultaneous operation of the WFC and SBC is not supported.
  2. The primary exposure must be taken with a filter from the list of primary/auto-parallel filter combinations ( Table 7.11). HRC and WFC share the same filter wheels, so the filter selection for the primary camera predetermines which filter will be used for the simultaneous observation in the other channel. (The HRC primary/WFC parallel filter combination will not always be the inverse of the WFC primary/HRC parallel combination, because the WFC and HRC optical paths are not diametrically opposite each other in the filter wheel).
  3. The primary exposure time must be greater than the minimum exposure time, defined in Table 7.12. These values are calculated such that all of the commanding associated with the auto-parallel is scheduled during the exposure time of the primary CR-SPLIT portion.
  4. The primary exposure is not allowed to have any EXPAND, MIN DUR, MAX DUR, or RT ANALYSIS special requirements.

Table 7.11: Filter combinations for auto-parallels in the two cases of (i) HRC camera prime and (ii) WFC camera prime.
HRC primary
WFC auto-parallel

WFC primary
HRC auto-parallel
CLEAR
CLEAR

CLEAR
CLEAR
F892N
F775W

F475W
F850LP
F606W
F625W

F658N
F550M
F502N
F550M

G800L
F625W
G800L
F850LP

F502N
F775W
F555W
F606W

F606W
F555W
F775W
F502N

F850LP
G800L
F625W
G800L

F550M
F502N
F550M
F658N

F625W
F606W
F850LP
F475W

F775W
F892N
F330W
F660N

F660N
F330W
F250W
F814W

F814W
F250W
F220W
F435W

F435W
F220W

Note that since the WFC and HRC apertures are not opposite each other on the filter wheels, filter pairs in columns 1 and 2 do not map to columns 3 and 4.

Table 7.12: Minimum primary exposure time (seconds) to attach an auto-parallel for CR-SPLIT=n.
Primary exposure type
Multiple auto-parallel scenario (n 1)
HRC
n × 495
WFC compressed
n × 406
WFC uncompressed
n × 465

(if CR-SPLIT=NO then n=1, if CR-SPLIT not specified n=2.)
Figure 7.9: Scheduling of auto-parallels.

 

There are a few things to remember about auto-parallels:

The following policies will be in effect for Auto-Parallel observations:

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 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 with ACS Coordinated and ACS Pure Parallels

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.


 7.8.1 Determining Orientation for Phase II  7.10 Pointing Stability for Moving Targets
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