NIRISS Imaging APT Template
Instructions for filling out the JWST NIRISS imaging template, using the Astronomers' Proposal Tool (APT), are provided in this article.
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See also: NIRISS Imaging, JWST Parallel Observations, NIRISS Imaging Recommended Strategies, APT Coordinated Parallel Observations
Imaging is one of 4 modes on the Near Infrared Imager and Slitless Spectrograph (NIRISS), offering image capability over the wavelength range from 0.8 to 5.0 μm. This mode is offered as a prime observing mode and as a coordinated parallel observing mode when NIRCam imaging or NIRCam wide field slitless spectroscopy (WFSS) is used as the prime science mode.
The user will have control over the following parameters: (1) Dithers, (2) Filters, and (2) Readout Pattern and exposure time (via Groups/Int and Integrations/Exp).
Step-by-step APT instructions are provided below.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
NIRISS Imaging tab
Choose the NIRISS imaging dither pattern. The dither offsets are large enough to ensure that a compact source does not fall on the same pixel twice, but not so large as to be affected by distortion across the detector.
- Image Dithers: number of steps, where the first step corresponds to initial pointing; allowed values are 2, 3, 4, 6, 8, 12, 16
- Pattern Size: amplitude of dither step; allowed values are SMALL (≈0.3"), MEDIUM (≈0.6"), and LARGE (≈1.2")
See also: NIRISS Filters, NIRISS Detector Readout Patterns, Understanding JWST Exposure Times
- Filters: F090W, F115W, F140M, F150W, F158M, F200W, F277W, F356W, F380M, F430M, F444W, F480M
- Readout Pattern
- NISRAPID: all frames are read and stored. Generally this can be used if the number of frames per integration is small and data volume is not an issue. There is a limit of 30 on the number of groups per ramp in the NISRAPID read out, though a limit of 25 is recommended to mitigate the effects of cosmic ray hits.
- NIS: four frames are averaged per group. Used for producing a lower data rate for longer integrations of faint targets.
- Groups/Int: the number of groups during an integration, equal to the number of frames read per integration (for NISRAPID), or the number of frames read out divided by 4 (for NIS).
- Integrations/Exp: the number of integrations during an exposure, where an integration is the time between detector resets.
- ETC Wkbk. Calc ID: the accompanying ETC workbook ID number for the calculation (optional).
The Exposure Time Calculator (ETC) should be used to determine the combination of Groups/Int and Integrations/Exp necessary to achieve the required signal-to-noise ratio. Individual integrations longer than 1,000 s are discouraged because of the effect of cosmic ray hits on the detector. For long exposures, a suitable number of integrations and a number of groups less than or equal to 25 is suggested.