NIRISS Imaging Recommended Strategies
Advice on how to optimize JWST NIRISS imaging coordinated parallel observations done with primary observations using another JWST instrument.
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 choosing exposure parameters, selecting aperture extraction sizes in the Exposure Time Calculator, and designing an efficient science program are given below.
Recommended Aperture Strategy Choices for Exposure Time Calculator
The Exposure Time Calculator (ETC) is used to calculate signal-to-noise ratios (SNRs) for an observation based on input exposure parameters. When determining exposure parameters in the ETC, users can select the aperture radius from which the flux is extracted and the background subtraction method. We recommended the filter-dependent source extraction radii listed in Table 1, which are based on the mean 80% encircled energy radii for a point source calculated from a WebbPSF grid. The recommended sky annulus for extracting the background region has an inner radius of 2 times the extraction radius and an outer radius of 4 times the extraction radius.
Table 1. Recommended aperture extraction radii for point sources for use in the ETC
Note: These extraction radii are for point sources. If the source is extended, it is up to the user to define the region of interest for calculating the SNR. It is also up to the user to ensure that other sources from the ETC scene are not included in the background extraction area, unless this effect is intended.
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 s. If the exposures in the primary instrument are relatively short, the NISRAPID readout 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 readout 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.
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.
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. For example, if one wishes to observe in the long wavelength wide filters F277W, F356W, and F444W (the latter two being similar to the WISE W1 and W2 filters) one should use the sequence F277W, F444W, and F356W rather than going in wavelength order.
It is not possible to predict where the FW and PW will be prior to an imaging parallel observation. In selecting the first filter of an imaging parallel observation one should bear in mind that if the previous NIRISS observation was in the AMI, WFSS, or SOSS modes that the FW will be more likely to be in the CLEAR/GR150R/GR150C positions, whereas the PW will be more likely to be in the NRM/GR700XD positions. Hence, starting with a filter near any of these positions is likely to save wheel moves.
One should generally carry out any short wavelength or long wavelength filter observations as a group rather than switching between these sets more than once in an observation. Within either the short wavelength or long wavelength group the order should minimize the wheel moves after the first filter. If one is observing in the short wavelength filters first, remember that the FW will be in the CLEAR position prior to any observations with the long wavelength filters. Conversely if one is observing in the long wavelength filters first, remember that the PW will be in the CLEARP position prior to any observations in the short wavelength filters.