Webb Science Instruments Overview

Nikole Lewis, nlewis@stsci.edu

The James Webb Space Telescope offers a broad range of observing modes covering a wide wavelength range from the optical to the mid-infrared (0.6 to 28.5 microns). These modes will offer unprecedented photometric and spectroscopic performance to enable a broad range of astronomical science. The four Webb science instruments (Near-InfraRed Imager and Slitless Spectrograph [NIRISS], Near InfraRed Camera [NIRCam], Near InfraRed Spectrograph [NIRSpec], and Mid-Infrared Instrument [MIRI]) offer complimentary capabilities including multiple imaging and spectroscopic modes. An overview of instrument coverage appears in Figure 1.

Figure 1. Overview of Webb Science Instrument wavelength coverage.

Imaging Modes

Standard Imaging

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

NIRISS is capable of imaging from 0.8 to 5 microns with a field of view of 2.2 × 2.2 arcmin. NIRCam is the camera of choice for near-infrared imaging with Webb, since it has twice the field of view of NIRISS and obtains "blue" and "red" images simultaneously. The NIRISS filter set closely match available NIRCam filters, which enables NIRISS to increase the resultant sky coverage when used in parallel with NIRCam.

MIRI provides broad-band (R ∼ 5) imaging from 5 to 27 microns over a field of view of 74 × 113 arcsecs, with a pixel scale of 0.11 arcsec.

Figure 2. Coverage detail for Webb imaging instrument modes.

Coronagraphy

For wavelengths between 1.7 and 5 microns, NIRCam has the capability to suppress the light from a bright target, with several different choices of 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 10-5 to 10-6 at 2 microns for a source separation of 1 arcsec.

For observations from 5–28 microns, 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 arcsec. It is intended for observations at ∼23 microns.

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 arcsec, with contrasts in the range 10–3–10–4 for a source separation of 1 arcsec.

Aperture-Masking Interferometry

Through the use of a non-redundant aperture mask (NRM), NIRISS provides Webb's highest-resolution imaging. The mask turns the full aperture of the telescope into an interferometric 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 arcsec with a brightness (contrast) ratio as small as 10–4–10–5. The resolution provided by the longest baseline is ∼0.075 arcsec at 4.6 microns. The NRM is optimized for observations through three medium-band filters from 3.5–5 microns, with observations possible as well through a broad-band filter at 2.8 microns.

Spectroscopic Modes

Figure 3. Coverage detail for Webb spectroscopic modes.

Dispersers

NIRSpec has one R ∼ 100 prism, three R ∼ 1000 gratings, and three R ∼ 2700 gratings. The prism covers 0.6–5.2 microns in one exposure or 1.0–5.2 microns with a long pass filter. The gratings are used in first order with four long pass filters to observe ∼0.7–1.2, ∼1.0–1.8, ∼1.7–3.0, or ∼2.9–5.2 microns without second-order contamination. Four exposures are required to cover 0.7–5.2 microns at R ∼ 1000 or R ∼ 2700.

NIRISS has two R ∼ 150 grisms that disperse in orthogonal directions and one R ∼ 700 cross-dispersed grism. The R ∼ 150 grisms cover 0.8–2.8 microns. The cross-dispersed grism simultaneously covers 0.9–2.8 microns at R ∼ 700 in first order and 0.6–1.4 microns at R ∼ 1400 in second order before orders start to merge at longer wavelengths. One exposure is required to cover the full wavelength range with each disperser.

NIRCam has two R ∼ 1450 grisms, in the long-wave channel, with orthogonal orientations. A grism is used with a broadband filter to observe 2.4–3.1, 2.4–4.0, 3.1–4.0, 3.9–4.3, or 3.9–5.0 microns. Two exposures are required to cover the full 2.4–5.0 micron wavelength range. Simultaneous photometry in the short-wave channel can be specified to complement long-wave channel grism observations.

MIRI has one R ∼ 100 prism and twelve R ∼ 1500–3200 gratings that cover 5–29 microns. The prism covers 5–12 microns in one exposure. Only three exposures are required to cover the full wavelength range at medium spectral resolution because three dichroics are used to record spectra from four gratings simultaneously.

Single-Object Spectroscopy

Two instruments have slits that can be used to observe a single object, while blocking light from other objects in the field of view. NIRSpec has five slits that can be used with any of the dispersers to observe a single object. Three of these slits are 0.2 × 3.3 arcsec in size, one is a 0.4 × 3.65 arcsec slit, and there is a 1.6 × 1.6 arcsec square aperture that is optimized for precise flux measurements of exoplanet host stars. MIRI has a 0.51 × 4.7 arcsec slit that can be used with the R ∼ 100 prism.

Three instruments have slitless configurations that can be used to observe a relatively bright isolated source, such as an exoplanet host star, without variable slit losses. The NIRISS cross-dispersed R ∼ 700 grism covers 0.6–2.8 in one exposure. One NIRCam R ∼ 1450 grism covers 2.4–5.0 microns in two exposures. The MIRI R ∼ 100 prism covers 5–12 microns in a single exposure. In all cases, contamination from neighboring objects may be an issue for certain targets at certain telescope orientations.

Multi-Object Spectroscopy

NIRISS can obtain slitless spectra of all objects in the field of view using the two R ∼ 150 grisms with orthogonal orientations and a direct image to facilitate spectral extraction. NIRCam has a similar capability at longer wavelengths (2.4–5 microns) and much higher spectral resolution (R ∼ 1450, hence much longer spectra).

NIRSpec has a microshutter array (MSA) with a quarter million configurable shutters. These MSA shutters can be opened in columns to form mini spectral slits on targets of interest. This allows multiplexing spectroscopy of dozens to hundreds of targets, depending on target catalog density and observation selection constraints. Each individual shutter is 0.2 × 0.46 arcsec. Because the cadence of shutters is fixed, objects will not all be centered in their respective shutters. Observers will use a specialized tool to plan MSA observations. The MSA can be used with any of the 7 NIRSpec dispersers, but at R ∼ 2700 spectra of objects near one edge of the field of view will be truncated.

Integral Field Spectroscopy

Two instruments have integral field units (IFUs). The NIRSpec IFU has an image slicing design with 30 0.1 arcsec slices that sample the 3 × 3 arcsec field of view. The NIRSpec IFU can be used with any of the 7 dispersers and is optimized for three-dimensional imaging spectroscopy of spatially extended targets. The MIRI IFU feeds the 12 gratings four at a time using three dichroics. The field of view increases from about 4 × 4 arcsec at short wavelength to about 8 × 8 arcsec at long wavelengths. The fields of view in each wavelength interval are nested.

Expected Performance

The Webb sensitivities reported in the plots below, in Figure 4, are the mission requirements: the minimum performance required of each instrument. This is a conservative approach and the on-orbit performance may be better. Expected performance of the instruments will be released in the Webb flight Exposure Time Calculator in January 2017. Sensitivities for other observatories are taken from instrument handbooks, online calculators, and published papers.

Figure 4. Webb instrument sensitivities.

For additional and most up-to-date information on Webb instruments, observing modes, and performance visit: https://jwst.stsci.edu/instrumentation. A series of lectures and webinars regarding instrumentation and observing capabilities is discussed in an article by Christine Chen in this Newsletter (vol. 33, issue 3).