NASA's James Webb Space Telescope

Imaging Modes

0.4 0.5 0.6 0.7 0.8 0.9 1 1.5 2 3 4 Wavelength (µm) 5 6 7 8 9 10 15 20 30 B V R I J H K L NIRISS MIRI NIRCam 20” x 20” Coronagraphy Standard Standard 2.2’ x 4.4’ FOV FOV FOV Pixel Scale Pixel Scale 74” x 113” 24” x 24” 0.11” 0.065” 0.065” 2.2’ x 2.2’ 0.032” 30” x 30”

Direct Imaging

Available for NIRCAM, NIRISS and MIRI

Illustration of the JWST direct imaging fields of view
Relative sizes of the JWST direct-imaging fields of view.

NIRCam has two modules, each with a field of view of 2.2x2.2 arcminutes. Within each module, the light is split with a dichroic and sent to a short-wavelength (0.6-2.3 μm) and a long-wavelength (2.4-5 μm), channel which observe simultaneously. The short-wavelength channel has a scale of 0.032" per pixel, which Nyquist samples the PSF at 2 microns.  The long-wavelength channel has a pixel scale of 0.065". Each channel has a selection of wide-, medium- and narrow-band filters.

NIRISS is capable of imaging from 0.8 to 5 μm with a field of view of 2.2x2.2 arcminutes. It uses a spare filter set from NIRcam and has a pixel scale of  0.065" per pixel, which matches the sampling of the NIRCam long-wavelength channel. In general, NIRCam is the camera of choice for near-infrared imaging with JWST, since it has twice the field of view of NIRISS and obtains "blue" and "red" images simultaneously. Used in parallel with NIRCam, NIRISS can potentially increase the resultant sky coverage at these wavelengths.

MIRI provides broad-band (R~5) imaging from 5 to 27 μm over a field of view of 74x113 arcseconds, with a pixel scale of 0.11".

Aperture-Masked Interferometry

Available for NIRISS

NIRISS Aperture-masked interferometry mode
NIRISS uses a 7-hole non-redundant mask in the pupil wheel to provide interferometric high-resolution imaging from 3-5 μm. The first panel shows the pupil mask, the center panel shows a simulated image and the last panel shows power-spectrum of the point-spread function in Fourier space.

Through the use of a non-redundant aperture mask (NRM), NIRISS provides JWST's highest resolution imaging. The mask turns the full aperture of the telescope into an inteferometric array such that each baseline (i.e., the vector linking the centers of two holes) is unique and forms fringes with a unique spatial frequency in the image plane.  This observing capability is particularly useful for high-contrast imaging of sources around bright stars, as well as for measuring the structural properties of the nuclei of galaxies and star-clusters. The NRM is designed to detect point sources that are separated by 0.1 - 0.5"  with a brightness (contrast) ratio as small as 10-4-10-5. The resolution provided by the longest baseline is ~0.075" at 4.6 μm. The NRM is optimized for observations through three medium-band filters from 3.5-5 μm, with observations possible as well through a broad-band filter at 2.8 μm.

Coronagraphy

Available for NIRCAM and MIRI

Illustration of Coronagraphic imaging modes
Illustration of JWST coronographic apertures. The NIRCam illustration shows the apodized occulting spots and wedges, along with the neutral-density squares used for target acquisition. The MIRI ilustration shows a simulated debris disk behind the Lyot occulter and simulations of the HR8799 planetary system in the 4-quadrant phase masks.

For wavelengths less than 5 μm, NIRCam has the capability to suppress the light from a bright target, with several different choices for apodized occulters. The coronagraphic observing mode includes Lyot stops to suppress the diffraction wings of the bright source, as well as neutral-density squares to allow accurate target acquisition on bright targets. Inner working angles are ~4-6 λ/D. A fiducial contrast ratio is 10-6 at 2 μm for a source separation of 1".

For observations from 5-28 μm, MIRI has four individual coronagraphs, one of which is based on the classic design of Lyot and three of which are based on four-quadrant phase masks (4QPMs). The classical Lyot coronagraph places an occulting spot in the focal plane to block the light from a bright point source from entering the instrument, resulting in an inner working angle of 2.1". It is intended for observations at ~23 μm.

The 4QPMs in MIRI use an optical element that retards the phase by π in two diagonally opposite quadrants. If a monochromatic source is placed exactly at the center of the resulting four-quadrant phase mask, the rejection is formally complete. Working with spectral bandpasses of ~10%, the 4QPMs have inner working angles of 0.3-0.5", with contrasts in the range 10-3  – 10-4   for a source separation of 1".