Since all of the scientific instruments are located at fixed positions in the telescope focal plane, it is possible to increase the productivity of HST by observing simultaneously with one or more instruments in addition to the primary instrument. Those additional observations are called parallel observations.
Since each instrument samples a different portion of the HST focal plane (see Section 2.2 of the HST Primer), an instrument used in parallel mode will normally be pointing at a "random" area of sky several minutes of arc away from the primary target. Thus parallel observations are usually of a survey nature. However, many HST targets lie within extended objects such as star clusters or galaxies, making it possible to conduct parallel observations of nearby portions of, or even specific targets within, these objects.
Parallel observations are never permitted to interfere significantly with primary observations; this restriction applies both to concurrent and subsequent observations. Some examples of this policy are the following:
Depending on whether a parallel observation is related to any specific primary observation, it is defined either as a coordinated parallel or pure parallel. Coordinated Parallel Observations are observations related to a particular primary observation in the same proposal. Pure Parallel Observations are unrelated to any particular primary observation (i.e., the primary observation is in another program). Investigators interested in proposing for parallels are encouraged to consult the Parallel Observations User Information Report, which contains details on how parallels are scheduled, completion rates for recent cycles, and other useful information.
Coordinated Parallel Observations must be marked in the 'Observation Summary' section of the proposal (see Section 8.15).
Coordinated Parallels use one or more instruments, in addition to and simultaneously with the prime instrument in the same proposal, e.g., to observe several adjacent targets or regions within an extended object. Proposals that include Coordinated Parallel Observations should provide a scientific justification for and description of the parallel observations. It should be clearly indicated whether the parallel observations are essential to the interpretation of the primary observations or the science program as a whole, or whether they address partly or completely unrelated issues. The parallel observations are subject to scientific review, and can be rejected even if the primary observations are approved.
Proposers are not allowed to add Coordinated Parallel Observations in Phase II that were not explicitly included and approved in Phase I.
Pure Parallel Observations must be marked in the 'Observation Summary' section of the proposal (see Section 8.15).
Proposals for Pure Parallel Observations may specify either specific or generic targets, although the latter are more common. In order to prolong the lifetime of the transmitters on HST, Pure Parallel Observations will be limited to a total of 300 orbits in Cycle 16. Appropriate scheduling opportunities will be identified by STScI.
Experience with Pure Parallel Observations over the last several years indicates that parallel observing programs should be kept simple, to allow scheduling with typical prime observing programs. Parallel programs requiring multiple successive orbits are less likely to schedule than those that can be executed in a single orbit. Due to resource constraints at STScI, only a limited number of Pure Parallel Programs can be accepted in each cycle.
For Cycles 13, 14 and 15, the Advanced Camera for Surveys (ACS) was the most heavily subscribed instrument for primary science observations. This is expected to be true for Cycle 16 as well. Therefore, proposers should be aware that opportunities to use the ACS for Pure Parallel Observations will be more limited than for other instruments, and propose accordingly.
A "Default" HST Archival Pure Parallel Program was in place from Cycle 7 to midway through Cycle 13. This program, which was discontinued during Cycle 13 to prolong the lifetime of the transmitters on HST, consists of observation types using each of the possible pure parallel instruments, and was used to obtain uniform, non-proprietary data sets for the HST Data Archive during parallel observing opportunities that were not allocated to Coordinated or Pure Parallel GO Observations. A description of the observations and additional information can be found on the HST Archival Pure Parallel Program Web Page. STScI encourages submission of Archival Research proposals to analyze data obtained as part of this program.
The Review Panels and TAC will select the best science and prioritize the parallel programs. All GO Pure Parallel Programs will have zero proprietary period. Pure Parallel Programs will not be carried over to the next cycle. No Pure Parallel programs were awarded time in Cycle 15.
When ACS is the prime instrument, WFC parallel observations are permitted with HRC in prime and vice versa (so called "auto-parallels"). The filter choice for auto-parallels is predetermined by the filter used in the primary observation (see the ACS Instrument Handbook for details). For this reason, auto-parallels are created by software and execute automatically as pure parallels. Even though the observer does not specify auto-parallels in his/her proposal, the parallel data are proprietary, exactly in the same way as the primary data. Whether or not auto-parallels can be created depends on the primary exposure.
The ACS/WFC and ACS/HRC may be used for either Pure or Coordinated Parallel Observations with any other instrument as prime.
The ACS/SBC may be used for Coordinated Parallel Observations with any other instrument as primary, but only if the telescope orientation is exactly specified and the field passes bright-object checking. The ACS/SBC may not be used for pure parallel observations.
The FGS cannot be used as a parallel instrument.
NICMOS may be used for either Pure or Coordinated Parallel Observations with any other instrument as prime. Observations that use different NICMOS cameras at the same time are considered parallel observations and must be requested in Phase I.
The WFPC2 may generally be used for either Pure or Coordinated Parallel Observations with any other instrument as prime.
The spacecraft computers automatically correct the telescope pointing of the primary observing aperture for the effect of differential velocity aberration. This means that image shifts at the parallel aperture of 10 to 20 mas can occur during parallel exposures.
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