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Last Updated  Mar 24, 2017


The imaging mode of JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) images its entire 2.2' × 2.2' FOV at ≈0.065"/pixel using 12 filters covering 0.8–5.0 μm.  This mode is only available in parallel or for calibrations.

Introduction

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.

 


Imaging exposures

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. 

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

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.

Figure 3: Transmission profiles for the NIRISS filters

 Transmission profiles for the NIRISS filters

Click on the image for a larger view.


Imaging 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.  

 


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 links

Near Infrared Imager and Slitless Spectrograph, NIRISS

NIRISS Overview

JWST Parallel Observing Strategies

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

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JWST Observatory and Instrumentation