NIRISS Imaging

The Near Infrared Imager and Slitless Spectrograph (NIRISS) imaging mode enables standard multi-wavelength imaging over the NIRISS detector's 2.2' × 2.2' field of view with an image scale of 0.065"/pixel, using a suite of 12 filters to cover the wavelength range 0.8–5.0 μm.  

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Imaging is a fundamental component of the wide field slitless spectroscopy mode of NIRISS. However, the imaging mode will only be used as a "prime" capability to collect data for instrumental calibrations.

Although most JWST imaging science programs will use NIRCam at near-IR wavelengths, NIRISS science imaging can be used effectively for parallel observations with other instruments. Currently, NIRISS imaging is only supported to be used in parallel when NIRCam imaging is used as the prime science modeParallel observations with NIRCam are particularly interesting because all the NIRISS filters (except for F158M) have essentially identical counterparts in NIRCam. Consequently, NIRISS effectively becomes a third channel of NIRCam when the two instruments are used in parallel.

Other prime+parallel combinations involving NIRISS imaging in parallel will be considered for implementation in cycle 2.



Imaging exposures

Main article: NIRISS Pupil and Filter Wheel
See also: NIRISS Filters, NIRISS Wide Field Slitless Spectroscopy

Target acquisition is not needed for the imaging mode.

Exposures with short wavelength filters in the pupil wheel (PW) are implemented in combination with 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 used in the PW. 

Figure 1. Elements in the NIRISS pupil and filter wheel used for short wavelength imaging

Elements in the NIRISS pupil and filter wheel used for short-wavelength imaging

Optical elements available for short wavelength imaging by NIRISS are indicated by green dashed circles. "Blue" filters in the PW are used in combination with the CLEAR element in the FW.
Figure 2.  Elements in the NIRISS pupil and filter wheel used for long wavelength imaging

Elements in the NIRISS pupil and filter wheel used for long-wavelength imaging

Optical elements available for long wavelength imaging by NIRISS are indicated by green dashed circles. "Red" filters in the FW are used in combination with the CLEARP aperture in the PW. The pupil alignment reference (PAR) and its support spiders reduce the throughput of the CLEARP element by ~16%.


Imaging filters

Main article: NIRISS Filters

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.

The PW carries four wideband (F090W, F115W, F150W, F200W) and two medium-band (F158M, F140M) filters, all of which are centered at wavelengths shorter than 2.3 μm.

The FW has three wideband (F277W, F444W, F356W) and three medium-band (F430M, F380M, F480M) long wavelength filters.

Figure 3. Transmission profiles for the NIRISS filters

 Transmission profiles for the NIRISS filters



Imaging subarrays

Main article: NIRISS Detector Subarrays

The NIRISS imaging mode will use the entire detector array to obtain direct images for the wide field slitless spectroscopy (WFSS) mode and parallel science observations. Calibration data will be taken using subarrays; details are available in the NIRISS subarrays page.



Imaging dither patterns

Main article: NIRISS Dithers

Dithering is important for direct imaging observations with NIRISS, in particular at wavelengths ≤3 μm where the PSF is signficantly undersampled. Details of available patterns for different modes are available in the NIRISS Dithers article.



References

Fullerton, A., 2016, ESAC JWST "On Your Mark" Workshop,
NIRISS: Imaging and Interferometry




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