APT Coordinated Parallel Observations

Coordinated parallel observations can be specified in APT for certain combinations of templates, and are defined within the same APT proposal as the primary observations. 

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See also: JWST Parallel Observations for an overview.

Parallel observing is a technique that allows more than one instrument to be simultaneously operated to collect data, thus maximizing the scientific return from JWST. However, because of various operational constraints, including data rate and data downlink restrictions, there needs to be a science-driven justification for parallel observing. A separate article, JWST Parallel Observations, describes the high-level considerations.

Science parallels come in 2 basic varieties, coordinated parallels and pure parallels, and they are handled differently in APT. This article concentrates on coordinated parallels.

Coordinated parallels

Coordinated parallel observations are requested in the same proposal as their primary counterparts since they're intended to directly support the science goals of the program. Its implementation depends on the requested template combinations and characteristics of the individual instruments being used together, as well as which instrument is considered primary and which is parallel.

Combined templates for coordinated parallels

Table 1 shows the observing template combinations that are approved for use in Cycle 2 of JWST operations. Support for other modes may be added in future cycles, and will be announced when they become available.

Table 1. Compatible observing template combinations

Ref no.Template combinationComments

MIRI Imaging – NIRCam Imaging

Either can be primary


Either can be primary


Either can be primary

NIRSpec MOS – NIRCam Imaging

NIRSpec MOS must be primary

NIRCam Imaging – NIRISS Imaging

NIRCam must be primary
6NIRCam WFSS – MIRI ImagingNIRCam must be primary
7NIRCam WFSS - NIRISS ImagingNIRCam must be primary
8NIRSpec MOS - MIRI ImagingNIRSpec MOS must be primary

Note: for options 1, 2, and 3, either instrument can be the primary; for options 4 through 8, only one of the instruments can be the primary. 

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

For any of the (primary) templates that permit coordinated parallels, the coordinated parallel option is invoked by checking the Coordinated Parallel box in the APT observation template GUI, then selecting the secondary (parallel) template option from the pull-down menu, as shown in Figure 1.
Figure 1. Selecting a coordinated parallel observation in the primary instrument observing template

Selecting a coordinated parallel observation in the primary instrument template

The footprints of each instrument in the JWST focal plane are offset from each other, as shown in Figure 2. Hence, parallel observations do not view the same location, but rather are offset. Also, th exact regions to be observed will depend on the observatory position angle at the time the observation is scheduled. Note that the "Y" axes indicated in the figure are the reference vectors for measuring aperture position angles (APAs) for each instrument. See JWST Position Angles, Ranges, and Offsets.

Figure 2. JWST instrument detector locations in the JWST focal plane and ideal coordinate systems

JWST Focal Plane with Instrument Detectors

JWST focal plane V2-V3 coordinate system with instrument fields of view shown as cyan rectangles. (For NIRCam, only the 2 long wavelength detectors are shown.) For one SIAF aperture on each detector, the right-handed ideal coordinate system is shown in red.

General considerations

    1. For coordinated parallel observations, every exposure of the primary instrument must have a parallel counterpart and vice versa. (This rule avoids mechanism moves with one instrument during an exposure with the other instrument. Such mechanism moves may in principle cause noticeable jitter.)
    2. Every exposure with the parallel instrument must have an exposure duration (exposure time plus overheads) that is less than or equal to that of the corresponding exposure duration of the primary instrument. (This rule is enforced by APT, which will give an error if the rule is violated.)
    3. Dither specifications are handled differently for NIRCam and MIRI. For NIRCam, a dither pattern is selected and applied to the (whole) observation. (For instance, multiple parallel exposures attached to the multiple primary visits/exposures of a NIRCam observation can only use a single dither pattern.) For MIRI, dithers are specific for a given filter choice, so in principle the dither patterns can change within a visit.
    4. Using NIRCam WFSS or NIRISS WFSS in coordinated parallel mode involves complications, because of the need for direct images before and/or after the WFSS grism exposure(s). This forces "extra" exposures onto the other instrument to provide primary exposures that these direct images can be attached to (more details provided below).

Details of specific combinations (given as primary + parallel)

NIRCam Imaging + MIRI Imaging

In the coordinated parallel mode, there is no change in the way that NIRCam primary dithers are selected. At the subpixel dither selection, you can choose to stay with the normal NIRCam options or choose any of a number of other "custom" dither patterns that provide a level of optimization of dither step sizes for the parallel instrument while retaining optimal pixel phase sampling for the primary instrument. A complication in this context is that the MIRI dither options are filter specific (to accommodate the significantly  varying PSF size of MIRI), but the selection made in APT applies to all exposures associated with the observation in question. Hence, some compromises are sometimes necessary for parallel observing. For example, you may need to choose a dither for a longer wavelength filter to accommodate some exposures, but that may be suboptimal for any parallel exposures in the same observation for which MIRI actually observes at shorter wavelengths. 

For MIRI Imaging in a "prime+parallel" combination, the exposures/dither parameter value can only be "1". 

MIRI Imaging + NIRCam Imaging

With MIRI imaging as prime, the primary dither selection is where a standard MIRI dither or a custom dither for coordinated parallels would be selected. As mentioned above, MIRI dithers are filter-specific. Hence, different MIRI-optimized dither patterns can be selected for each individual NIRCam parallel exposure specification. (In other words, the NIRCam parallels may not all be done with similar dithers, unlike in the "NIRCam prime + MIRI parallel" case.)

NIRCam Imaging + NIRISS Imaging

NIRISS imaging is not offered as a primary mode in coordinated parallels, and so this pairing is always NIRCam primary. Similar to the "NIRCam imaging + MIRI imaging" combination, you can choose standard NIRCam dither options as well as a number of custom dither patterns that provide optimized PSF sampling for NIRISS and NIRCam simultaneously.

NIRCam Imaging + NIRISS WFSS, MIRI Imaging + NIRISS WFSS (or vice versa)

NIRISS WFSS observations (whether in primary or parallel) require a direct image before and after the grism exposures (with a given grism and blocking filter). This complicates matters in coordinated parallels because it requires that "prime + parallel" sets of exposures come in groups of 3 exposures instead of one. Specifically, each set of exposure sequences specified in the NIRISS WFSS template (consisting of a line of grism exposures plus a line of direct image exposures) requires 3 exposure specifications with the other instrument (NIRCam or MIRI). Of these 3, the 2nd exposure specification will be associated with the (dithered, typically relatively long) grism exposures of NIRISS WFSS, while the first and 3rd exposure specifications will be associated with 2 (typically relatively short) direct images of NIRISS WFSS. By default, the latter 2 exposures are single (undithered), and taken at the first and last dither position of the primary instrument's dither pattern. However, if NIRISS WFSS is the parallel instrument, the user has the option of imposing the full dither pattern of the primary instrument also on the direct images of NIRISS WFSS (this option is available in APT by setting the Direct Image Exposures pull-down selector to DITHER_DIRECT_IMAGES). 

The best way to accommodate this strategy is to add 2 extra (otherwise unnecessary) exposure lines under which you will specify the NIRCam or MIRI imaging exposures associated with the first and 3rd exposures in the NIRISS WFSS sequence (i.e., the WFSS direct images). This breaks one of the key tenants of parallel observing, which is that parallel observations should not impact the primary observations, but it is the only way to properly accommodate NIRISS WFSS in coordinated parallel mode. In terms of dithers, you can choose all standard dither patterns for the prime instrument as well as a number of custom dither patterns that provide optimized PSF sampling for both the prime and parallel instruments. A PDF file is available to show you how to create a set of "prime+parallel" exposures using the NIRCam imaging APT template, and using NIRISS WFSS in coordinated parallel mode.

NIRCam WFSS + MIRI Imaging, NIRCam WFSS + NIRISS Imaging

These cases are similar to the combinations involving NIRISS WFSS mentioned above, with the following differences:

  1. Direct images for NIRCam WFSS are optional rather than mandatory, except for the last exposure specification in the observation, in which case they are mandatory.
  2. Direct images for NIRCam WFSS are (only) taken after the grism exposures (with a given grism and SW/LW filter set), and they come in 2 separate sets (one called "Direct Image", the other called "Out of Field" images).

As such, each set of exposure specifications in the NIRCam WFSS template requires either 1 or 3 exposure specifications of the parallel instrument mode (MIRI imaging or NIRISS imaging). The first exposure specification of the parallel instrument mode will be associated with the dithered (and typically relatively long) grism exposures of NIRCam WFSS, while the (optional) second and 3rd exposure specifications of the parallel instrument mode will be associated with (typically relatively short) direct images of NIRCam WFSS. The latter (optional) exposures will be taken at 3 different locations on the sky (for details on that, see the NIRCam WFSS article).

NIRSpec MOS + NIRCam Imaging

This combination is only offered with NIRSpec MOS as the primary so it drives the pointing. If this coordinated parallel combination is selected, the NIRSpec MOS template includes the option to add customized subpixel dithers to the nominal moves of the field of view by integer shutters of the micro-shutter assembly (MSA; i.e., the "nods" in the MSA Planning Tool). The sizes of these subpixel dithers are chosen to improve NIRCam image pixel sampling while staying small enough to avoid incurring significant slit losses with the NIRSpec MOS exposures.

NIRSpec MOS + MIRI Imaging

This combination is also only offered with NIRSpec MOS as the primary, which drives the pointing. There is no option to add customized dithers for this coordinated parallel combination, because the nominal moves of the field of view by integer MSA shutters (i.e., the "nods" in the MSA Planning Tool) already provide adequate pixel sampling for MIRI imaging, and small subpixel dithers would not add any significant improvement to the data quality of the resulting MIRI Images. The spatial offsets in MIRI pixels associated with nominal nods between MSA shutters are tabulated below in Table 2.

Table 2. MIRI spatial offsets associated with NIRSpec nods between MSA slitlet shutters

MSA shutter setupMIRI pixel offsetsMIRI pixel phases
2 shutter slitlet(0.000, 0.000), (3.363, 3.192)(0.000, 0.000), (0.363, 0.192)
3 shutter slitlet(0.000, 0.000), (3.363, 3.192), (-3.363, -3.192)(0.000, 0.000), (0.363, 0.192), (0.637, 0.808)

5 shutter slitlet (shutters 1, 3, 5)

(0.000, 0.000), (3.363, 3.192), (6.725, 6.384),

 (-3.363, -3.192), (-6.725, -6.384)

(0.000, 0.000), (0.363, 0.192), (0.725, 0.384),

(0.637, 0.808), (0.275, 0.616)

Latest updates
    Updated for Cycle 2

    Updated to include descriptions for Coordinated Parallel combinations implemented in APT 2020.1.

  • Updated for APT 27.1 release

    Updated with current capabilities being offered for Cycle 1

  • Previously called "JWST APT Parallels." This article is specific to coordinated parallels. Pure parallel section was split off to a separate new article.

  • Added some “warning” information related to APT 25.1
Originally published