Last Updated Mar 24, 2017
The NIRISS imaging mode enables standard multiwavelength 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.
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. For example, parallel imaging observations with NIRISS can be used to provide:
- complementary wavelength coverage when MIRI is the prime instrument.
- increased sky coverage when NIRCam is prime.
Parallel 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.
Target acquisition is not needed for the imaging mode.
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.
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 4 wide-band (F090W, F115W, F150W, F200W) and 2 medium-band (F158M, F140M) filters, all of which are centered at wavelengths shorter than 2.3 μm.
The FW has 3 wide-band (F277W, F444W, F356W) and 3 medium-band (F430M, F380M, F480M) long wavelength filters.
Imaging dither patterns
Dithering is important for direct imaging observations with NIRISS, in particular at wavelengths ≤3 μm where the PSF is signficantly undersampled. Dithering for NIRISS direct imaging will make use of line or diamond shaped dither patterns with 2-4 dither steps.
Related linksFullerton, A., 2016, ESAC JWST "On Your Mark" Workshop,
NIRISS: Imaging and Interferometry
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