NIRISS Imaging Recommended Strategies

Imaging is not supported as a prime observing mode for NIRISS but is offered as a coordinated parallel mode when another JWST instrument is prime. Advice is given on how to optimize exposure specifications for NIRISS imaging coordinated parallel observations.

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Main article: NIRISS Imaging, NIRISS Imaging APT Template
See also: JWST Parallel Observations, JWST APT Coordinated Parallel Observations

NIRISS imaging is only offered as a coordinated parallel observation when another JWST instrument is prime. For Cycle 1, NIRISS imaging is only supported as a parallel observing mode when NIRCam imaging is the prime mode. This combination increases the areal coverage of NIRCam imaging observations. Other prime + parallel combinations involving NIRISS imaging will be considered in future cycles.

Guidelines for designing an efficient science program and choosing exposure parameters are given below.

Designing an efficient science program with multiple filters

Main article: NIRISS Pupil and Filter Wheels

NIRISS has 12 filters in the pupil (PW) and filter wheels (FW) that collectively cover the wavelength range between 0.8 and 5.0 μm (Figure 1). Exposures with short wavelength filters in the pupil wheel (PW) are implemented by selecting the CLEAR aperture in the filter wheel (FW). For imaging with any of the long-wavelength filters located in the FW, the CLEARP aperture is selected in the PW. 

When observing in multiple filters, choose an exposure sequence that minimizes mechanism moves between filters.

Figure 1. Layout of optical elements in the pupil filter and filter wheel

Layout of the NIRISS Pupil Wheel and Filter Wheel

Schematic diagram showing the layout of optical elements in the Pupil Wheel (left) and Filter Wheel (right). To design an efficient coordinated parallel imaging program, it is recommended to choose filters in the Filter Wheel and Pupil Wheel in sequential order so that mechanism moves are minimized.

When to Use NIS versus NISRAPID readout

Main article: NIRISS Detector Readout Patterns

The use of NIS readout is recommended if the individual exposures are longer than about 300 seconds. If the exposures in the primary instrument are relatively short, the NISRAPID read-out pattern is recommended to ensure that the best sampling is obtained in the parallel observations.  In particular if the NIRISS observations are in parallel to NIRCam BRIGHT1 or BRIGHT2 observations the best choice is to use NISRAPID read-out for NIRISS and match the integration time to the primary NIRCam integrations.

Note that when using the NISRAPID readout mode, there is a limit of 30 groups in an integration. While the Astronomer's Proposal Tool (APT) allows up to 200 groups for the NIS readout pattern, it is recommended to limit the number of NIS groups to 25 to mitigate cosmic ray hits on the detector.

Persistence considerations

Main article: NIRISS Bright Limits

For parallel observations, one cannot make any program adjustments that will mitigate the persistence.  The only consideration is whether there are very bright objects that may fall within the NIRISS field of view, which may affect the images in some other part of the field of view after dithering.  As a general guideline, stars brighter than magnitude 15 in the short wavelength filters and brighter than 12.5 in the long wavelength filters will saturate in the first few groups.  Hence if many stars of this brightness are present in the area, or if stars that are many magnitudes brighter than this may appear in the field of view, this is likely to cause persistence issues.  The WISE W1 and W2 filter images are useful as guides to assess the brightness of objects in the NIRISS long wavelength filters, while the 2MASS images are useful as guides to assess the brightness of objects in the NIRISS short wavelength filters.



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