JWST Parallel Observations
Some capabilities to use more than one science instrument simultaneously (in parallel) will be available for JWST Cycle 1. Additional instrument combinations may be offered in future cycles.
Parallel observing refers to simultaneously operating more than a single science instrument, each viewing a different area of the JWST focal plane. For JWST, there will be 2 basic modes of parallel operations: coordinated parallels and pure parallels.
Coordinated parallel observations are planned as part of a primary program, and are intended to amplify or supplement the primary science proposed in a given proposal. That is, the primary and parallel observations are planned together in a single program to accomplish the science goals, and hence are under the purview of a given proposal PI and science team.
Pure parallel observations are from programs that make use of parallel observing slots derived from other accepted proposals. Hence, pure parallel programs are separate proposals, have separate PIs, and are selected independently from any primary programs they may be attached to. It is expected that many calibration observations will be obtained using pure parallel opportunities, but there will be openings for pure parallel science observations as well.
There are a number of subtleties related to each of these modes of operation. For example, pure parallel observations are not permitted to impact or change the primary observations to which they are attached (with one exception described below), whereas coordinated parallel observations can be tailored to accommodate both the primary and parallel datasets since they are specified within a given proposal. (For example, dither patterns that work well with both instruments can be selected.) These subtleties are described below.
The goal of enabling parallel observing is to enhance the efficiency and science return of the JWST mission. However, users need to be aware that there are tradeoffs that need to be balanced. Not every potential parallel observing opportunity can be used for science. One example is the limited data downlink capability available to the observatory. For example, running NIRCam by itself in certain readout modes for several hours can use the entire data volume downlink capacity for a single contact period, leaving no room for storing parallel data taken during that period. Another consideration (for pure parallels) is that many calibration activities need to use pure parallel observing slots, and parallel calibration activities may take precedence over pure parallel science.
Only certain modes and combinations of instruments will be allowed for cycle 1 proposals (see below). Additional combinations may be made available in future observing cycles.
Principles in the use of parallel observing with JWST
A detailed description of the policies related to JWST parallel observing is available. In summary, there are 3 principles that govern the policies for proposing and planning parallel observations:
- For coordinated parallels, the science goals of the parallel observations must be tightly linked to the science goals of the overall program. In other words, accomplishing the main goals of the proposal requires parallel observations. Coordinated parallels whose goal is to address ancillary topics will generally not be approved.
- Pure parallel observations may not impact the primary observations they are attached to†. This means pure parallel exposures must fit under the resource footprint (exposure plus overheads) of the primary exposure to which they are attached.
- The needs of the JWST calibration program, a large fraction of which takes place in parallel mode, take precedence over pure parallel science programs.
† As a practical matter, adding pure parallels will add a small additional overhead for setting up the instruments. This will be handled by the scheduling system and will not be assessed post-facto as an overhead on the primary observations. Further discussion of policies can be found in the Call for Proposals.
Main article: APT Coordinated Parallel Observations
See also: JWST Position Angles, Ranges, and Offsets, JWST Background Model, JWST Data Rate and Data Volume Limits, JWST General Target Visibility Tool Help
Coordinated parallel observations are crafted within the APT template used for the primary instrument mode. Five template combinations were supported at the time of ERS proposing, but 3 additional modes have beed added for Cycle 1, as shown in Table 1.
Table 1. Template combinations supported for ERS (1-5) and cycle 1 observations
|Ref. no.||First (Primary) template||Second (Parallel) template||Comments|
|Either template can be selected as primary, with the other as parallel.|
|2||NIRCam imaging1||NIRISS WFSS||Either template can be selected as primary, with the other as parallel.|
|3||MIRI imaging||NIRISS WFSS||Either template can be selected as primary, with the other as parallel.|
|4||NIRCam imaging1||NIRISS imaging||NIRCam must be primary. Use to increase areal coverage, but note NIRISS differences in pixel size and available filters.|
|5||NIRSpec MOS||NIRCam imaging||NIRSpec MOS must be primary.|
|(Modes added January 2020)|
|6||NIRCam WFSS||MIRI Imaging||NIRCam WFSS must be primary.|
|7||NIRCam WFSS||NIRISS Imaging||NIRCam WFSS must be primary.|
|8||NIRSpec MOS||MIRI Imaging||NIRSpec MOS must be primary.|
1 Only direct imaging with standard narrow-, medium-, or broadband filters is allowed for NIRCam and MIRI observations in these coordinated parallel modes.
Instrument teams have worked with developers to provide several ways of scheduling joint observations in parallel, for instance, to designate one instrument as the primary, as shown in options 1–3 in Table 1.
One anticipated use case is to use both instruments at one epoch to observe adjacent areas of the sky, and to return at a second epoch when the instrument fields of view have rotated 180° on the sky. Users should obviously investigate the availability of the relevant position angles using the JWST target visibility tools since not all positions on the sky have the needed flexibility in available position angles to accommodate this strategy. Users should also be aware that background levels due to zodiacal emission and thermal emission from the telescope will change between two observations at different position angles. However, other science cases may not require such a "180° strategy" and can simply obtain observations (e.g., with NIRISS WFSS) of a nearby areal region to accompany NIRCam or MIRI primary imaging observations.
All dither patterns available for a given instrument template will also be available when it is selected as the primary instrument in coordinated parallel mode. However, you also have the option to select from sets of customized dither patterns that have been specifically designed to produce good results for both instruments simultaneously. The number and type(s) of customized dither patterns that are available depend on the specific prime+parallel instrument mode combination. When a parallel instrument is selected, these additional dither patterns will become available in the dither pull-down menu in the relevant APT template.
Issues regarding data volume per downlink period should also be considered, especially for options involving NIRCam. Since NIRCam observations involve up to 10 detectors (8 short wavelength and 2 long wavelength), proposers of coordinated parallel programs may need to consider selecting readout patterns that are less data intensive than they might otherwise choose. (Some of the most data intensive readout patterns for NIRCam are disallowed by APT in parallel mode.) Alternatively, depending on the science use case, a user may decide that areal coverage is less important than the parallel coverage and may opt to use only one of the two NIRCam modules. APT will compute and display the data volume for each exposure, and you can adjust the detector readout pattern and/or the number of modules being read out as needed to stay within the allowed limits.
A number of additional coordinated parallel options are being considered for future cycles, including the possibility of using up to 3 instruments simultaneously.
Main article: APT Pure Parallel Observations
Unlike coordinated parallels, pure parallel observations are proposed as entirely separate programs of investigation. Pure parallels use parallel observing "slots" created by exposures from other accepted programs that do not already have parallel observations specified. Pure parallel observations will not be allowed to influence the dither patterns or other aspects of the observing strategy of the primary observations to which they are attached, since the primary observations will belong to different science proposals. There are 4 observing modes supported for pure parallel observations in Cycle 1:
Not all observing templates will be allowed to host pure parallel observations, for various technical reasons or concerns about how a pure parallel observation could potentially impact the data quality of the primary.
Templates that are allowed to have pure parallels attached are the following:
NIRCam imaging, NIRCam WFSS, MIRI imaging, NIRSpec MOS, NIRSpec IFU, NIRSpec fixed slits, NIRISS WFSS, MIRI MRS, and MIRI LRS (the latter only if Subarray = FULL).
Templates that are not allowed to have pure parallels attached are the following:
NIRCam time series, NIRCam grism time series, NIRCam coronagraphy, NIRSpec bright object time series, NIRISS AMI, NIRISS SOSS, MIRI coronagraphy, and MIRI LRS (if Subarray = SLITLESSPRISM).
The NIRISS WFSS mode requires direct imaging exposures before and/or after the dispersed grism spectral exposure to obtain images of the undispersed field. Hence, to obtain such a set of exposures in a pure parallel mode, appropriate double or triple slots will need to be identified to attach these observations.