The James Webb Space Telescope Mission

JWST has four science instruments with a total of 17 science instrument modes (see Table 4). All of the instrument capabilities expected before launch have been enabled and are in use. Almost all of the instrument requirements have been exceeded; in particular most of the instrument modes are more sensitive than the pre-launch expectations, and the point spread function at the shorter wavelengths is sharper than the pre-launch expectations.

V.1 NIRCam

NIRCam [108, 57, 109, this issue] provides imaging from 0.6μm to 5.0μm in broad-band, medium-band and narrow-band filters. It has a wide-field slitless spectroscopy capability from 2.5μm to 5.0μm. It has a coronagraphic mode. NIRCam is designed with two modules observing parallel fields of view; each module contains a dichroic at 2.4μm to provide simultaneous data in two filters longward and shortward of the dichroic. The two modules are identical, including the wavefront sensing hardware, and provide full redundancy in case of failure. The total field of view is 2.2×4.4arcmin², and there are a total of ten detectors, with 8 in the short wavelength channel and 2 in the long wavelength channel. All of the near-infrared instruments (NIRCam, NIRSpec, NIRISS, and FGS) use H2RG HgCdTe 2048×2048 focal-plane arrays made by Teledyne Imaging Systems [103, e.g.,]. The NIRCam plate scales are 32 milli-arsec per pixel in the short wavelength channels and 65 milli-arcsec per pixel in the long wavelength channels, Nyquist sampling the diffraction limit at 2.0μm and 4.0μm, respectively. NIRCam also functions as part of the wavefront sensing and control system.

V.1.1 NIRCam imaging

NIRCam contains 2 extra-wide filters, 8 broad-band filters, 12 medium-band filters and 7 narrow-band filters. The broad-band filters span the full wavelength range of the instrument, the extra-wide and medium-band filters cover 1.0μm to 4.0μm and 1.4μm to 5.0μm respectively, and the narrow-band filters are selected to match individual spectral lines. NIRCam imaging sensitivity exceeds the pre-launch expectations in almost all filters. The requirements were 11.4nJy and 13.8nJy at 2.0μm and 3.5μm, point-source sensitivity, 10σ in 10,000s. The sensitivity at the end of commissioning were 7.3nJy and 8.8nJy respectively. NIRCam imaging is one of the most-used modes in Cycle 1 programs. An example program that uses this mode is Program 1963, a medium-band survey of the Hubble Ultra Deep Field.

V.1.2 NIRCam wide-field slitless spectroscopy

NIRCam wide-field slitless spectroscopy [52, WFSS] provides R∼1600 spectra of all of the objects within the field of view using a grism. The grism is used in the long wavelength channel in combination with a wide or medium filter to provide spectroscopy in the 2.5μm to 5.0μm wavelength range. Short wavelength <2.5μm imaging in the short wavelength channel can be taken simultaneously with the WFSS measurements. Commissioning data show that the total throughput is 20% to 40% higher than pre-launch expectations. An example using this mode is Program 2078, searching for galaxies at the same redshift as 6.5<z<6.8 quasars. Sun et al. [127], Sun et al. [128] discovered Hα+[O III] λ5007 line emitters at z>6 using the NIRCam WFSS mode in commissioning data.

V.1.3 NIRCam coronagraphy

NIRCam coronagraphy has 3 round- and two bar-shaped coronagraphic masks for occulting a bright object. The inner working angles range from 0.40″ to 0.81″ for the round masks, corresponding to 6λ/D at 2.1μm, 3.35μm and 4.1μm, and 0.13″ to 0.88″ for the bar masks. During a bar observation, the bright object is positioned behind the bar at the location where the IWA ∼ 4λ/D. The masks are used in conjunction with a filter. Commissioning demonstrated that this mode provided a 5σ contrast at 1″ better than 4×10⁻⁵. An example using this mode is Program 1386, the DD-ERS high-contrast exoplanet imaging program.

V.1.4 NIRCam bright object time series - imaging

Bright object time series (BOTS) observations are designed to measure photometric variations in relatively bright sources. NIRCam imaging BOTS uses rapid readout of sub-arrays ranging from 64×64 to 160×160, in combination with filters or a weak lens, to increase the readout cadence and increase the saturation limits. Commissioning observations showed that the NIRCam BOTS imaging performance was nominal. An example using this mode is Program 2635, studying infrared emission of 4U0142+61, a magnetar with a possible silicate spectral feature at 9.7μm, which has been interpreted as a passive disk surrounding the energetic isolated neutron star.

V.1.5 NIRCam bright object time series - grism

NIRCam grism BOTS observations provide R∼1600 spectroscopic observations of bright, isolated, time-varying sources. The spectroscopy in the long-wavelength channel is paired with weak lens observations in the short-wavelength channel to avoid saturation. This mode is capable of observing targets as bright as naked-eye stars (mag < 5). Commissioning observations of the transiting exoplanet HAT-P-14 b obtained a 91 ppm spectrum (when binned to R=100). An example using this mode is Program 2084, searching for lava rain on the hot super-Earth planet 55 Cancri e.

V.2 NIRSpec

NIRSpec [61, 15, this issue] provides spectroscopy from 0.6 to 5.3 μm at R∼100, R∼1000 and R∼3000 using fixed slits, a microshutter assembly (MSA), or an integral field unit (IFU). The detector system consists of two Teledyne 2048 × 2048 H2RG arrays controlled and read by SIDECAR ASICs. The 18μm×18μm pixels of the detector arrays project to an average of 0.103″ in the dispersion direction and 0.105″ in the spatial direction. Dispersion is done with a prism (R=30−300) or gratings (R=500−1343 or R=1321−3690). The dispersion is crossed with filters to limit the bandwidth and resulting length of the spectra on the detectors. In most cases, the throughput of the instrument is higher than pre-launch expectations.

V.2.1 NIRSpec multi-object spectroscopy

NIRSpec multi-object spectroscopy is done using the MSA, which is a MEMS assembly consisting of four quadrants. Each MSA slit is 0.203″ by 0.463″, with a ∼0.07″ wall between the openings. There are 730 (spectral) by 342 (spatial) pixels in the full array, spanning a field of view of approximately 3.6′ by 3.4′. The MSA is fully configurable, except for a limited number of failed slits. Typically, the MSA can be configured to observe up to 100 objects simultaneously, including sky subtraction, without overlapping spectra. Multi-object spectroscopy is one of the most highly used instrument modes in Cycle 1. An example using mode is Program 1345, The Cosmic Evolution Early Release Science (CEERS) Survey, the DD-ERS program targeting a deep field.

V.2.2 NIRSpec fixed slit spectroscopy

NIRSpec has five fixed slits, which provide the highest contrast and throughput on individual targets for NIRSpec. The fixed slits also provide a redundant spectroscopic capability to the mission if the MSA mechanism were to fail. Three fixed slits are 0.2″ wide by 3.3″ long, and one is 0.4″ wide by 3.8″ long. There is also a 1.6″×1.6″ high-throughput slit that is primarily used with the NIRSpec bright object time series mode. An example using this mode is Program 1936, a target of opportunity program which will target a kilonova detected by the LIGO/Virgo/KAGRA gravitational wave detectors during their Observing Run 4.

V.2.3 NIRSpec integral field unit spectroscopy

The IFU entrance aperture is a contiguous 3.1″×3.2″ field of view, divided into 30 slices totaling 900 spaxels, each 0.103″×0.105″. By providing a full spectral data cube, the mode gives the most complete information on a single target in the near-infrared with JWST. The throughput is slightly lower than pre-launch expectations in the red, but higher in the blue. An example using this mode is 1355, Targeting Extremely Magnified Panchromatic Lensed Arcs and Their Extended Star formation (TEMPLATES), the DD-ERS program that targets individual galaxies that are highly boosted by gravitational lensing.

V.2.4 NIRSpec bright object time series

NIRSpec BOTS primarily uses the 1.6″×1.6″ fixed slit, combined with detector sub-arrays either 16 or 32 pixels wide, to rapidly monitor bright time-varying objects such as observations of stars with transiting exoplanets. The readout cadence can be as fast as 0.28s, potentially reaching stars brighter than J<6 in some modes. During commissioning, the mode was tested on HAT-P-14 b, and reached a noise level of <60 ppm. An example using this mode in Cycle 1 is Program 2159, following a hot super-Earth-size exoplanet for a full orbit to map the planet’s temperature.

V.3 NIRISS

NIRISS [30, 29, this issue] provides three specialized scientific capabilities and redundant broad-band imaging, over the wavelength range 0.7μm to 5.0μm. NIRISS is packaged with the FGS, which provides the signal to the fine steering mirror and the attitude control system to lock onto targets and provide fine guiding. NIRISS has a field of view of 2.2′×2.2′, matching the FOV of one of the two NIRCam channels. NIRISS has a single 2048×2048 5μm cut-off Hawaii-2RG detector, with 65 milliarcsec per pixel.

V.3.1 NIRISS single object slitless spectroscopy

NIRISS single-object slitless spectroscopy (SOSS) (Albert et al., in prep) defocuses the telescope beam to spread the signal from bright objects over about 25 pixels to avoid saturation. SOSS provides medium-resolution spectroscopy (R∼70) between 0.6μm and 2.8μm. There are two usable orders. Using subarrays shortward of 1.0μm allows targets as bright as J=6.5 (Vega mag). An example is Program 2589, where the SOSS mode will be used to detect and characterize the possible atmospheres of the small, rocky exoplanets TRAPPIST 1b and 1c.

V.3.2 NIRISS wide field slitless spectroscopy

NIRISS wide-field slitless spectroscopy (WFSS) enables low-resolution (R∼150) slitless spectroscopy over the 2.2′×2.2′ FOV at 0.8μm to 2.2μm wavelength. It is optimized to search for Ly-α emitting galaxies during the epoch of reionization. It can also be used efficiently in parallel mode. Two orthogonal gratings provide dispersion in two directions to disentangle overlapping spectra and reduce confusion in crowded fields. The gratings are crossed with wide- or medium-band filters, which also reduces blending of the objects. Throughput of the WFSS mode exceeds the pre-launch expectations. An example is Program 1571, PASSAGE – Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution, a pure-parallel search for active star-forming galaxies.

V.3.3 NIRISS aperture masking interferometry

NIRISS aperture masking interferometry [119, 120, AMI,] uses a seven-aperture mask to enable high-contrast imaging at an inner working angles less than λ/D. AMI is used with the F380M, F430M, or F480M filters, and typically uses an 80×80 pixel subarray for bright sources. During commissioning, AMI was demonstrated by detecting AB Dor C, a companion separated by ∼0.3″ with a contrast ratio of 4.5 mag. An example is the DD-ERS Program 1349, WR DustERS, which will observe the Wolf-Rayet binary WR 137 with AMI to investigate the dust abundance, composition, and production rates of dusty sources in the colliding winds of the stars.

V.3.4 NIRISS imaging

NIRISS includes an imaging capability using a set of backup NIRCam broad-band filters and some medium-band filters. As NIRISS imaging covers half the FOV of NIRCam, and does not include a dichroic, this mode is primarily for imaging redundancy in the mission. NIRISS imaging can also be used in parallel to NIRCam imaging for additional areal coverage, and is used in support of WFSS data. Program 2561, which will observe the Frontier Field lensing cluster Abell 2744, uses NIRISS imaging in parallel to NIRCam imaging to increase the area of deep photometric studies of high-redshift galaxies at mild lensing magnifications.

V.4 MIRI

MIRI [106, 144, 143, this issue] provides both imaging in broad-band filters and IFU spectroscopy from 5.0μm to 28.0μm. It also has low-resolution slit spectroscopy from 5.0μm to 12.0μm (where the sensitivity is limited by the zodiacal light background) and a coronagraphic capability. MIRI has three arsenic-doped silicon (SI:As) impurity band conduction detector arrays, each of 1024×1024 pixel format with 25μm pixel pitch, made by Raytheon Intelligence & Space. The plate scale is 110 milli-arcsec per pixel, which Nyquist samples the point spread function at 6.25μm. Two of the detectors are used for the medium-resolution spectroscopy, while the third is used for imaging, low-resolution spectroscopy and coronagraphy. The MIRI instrument is actively cooled to an operating temperature of 6.0K with a ∼6K/18K hybrid mechanical cooler, developed by Northrop Grumman in collaboration with JPL. The MIRI cooler uses gaseous helium as the coolant. There is a three stage Pulse-Tube Precooler which reaches ∼18K and a fourth ∼6K Joule-Thompson cooler stage. The cooler compressor is in the JWST spacecraft bus at room temperature, while the cold head assembly cooling the instrument is on the ISIM structure.

V.4.1 MIRI imaging

MIRI imaging uses 9 broad-band filters to cover the 5μm to 27μm wavelength region. The imaging field of view is 1.4×1.9arcmin² sampled with 0.11″ pixels. (The remaining field of view of the detector is occupied by the coronagraphs and the low-resolution spectrometer.) MIRI imaging can use sub-array readouts for bright objects that would saturate in a full frame, observing objects as bright as 0.1Jy in the F560W filter. An example using this mode is Program 2130, which will observe several square kpc in three nearby galaxies, M33, NGC 300 and NGC 7793, to measure dust-enshrouded stellar populations.

V.4.2 MIRI low-resolution spectroscopy

MIRI low-resolution spectroscopy [65, LRS] provides R∼100 long-slit and slitless spectroscopy from 5μm to 12μm, the MIR wavelength range where JWST observations are still zodiacal-light limited. A slit mask is permanently in the field of view; slitless spectroscopy is available anywhere within the imager field of view when the R∼100 double prism assembly is selected in the imaging filter wheel. For bright sources, a sub-array readout can be used. In practice, the source will be placed in a dedicated LRS slitless detector region and read out in a sub-array. It is expected that most LRS slitless targets will be bright nearby stars with transiting planets to obtain spectra of exoplanet atmospheres. Program 1658 will observe Pluto’s moon Charon using the MIRI LRS mode.

V.4.3 MIRI medium-resolution spectroscopy

MIRI medium-resolution spectroscopy [139, MRS] provides integral-field spectroscopy over the full 5μm to 28μm MIRI wavelength range. The spectral resolution ranges from ∼3300 at the short wavelength end to ∼1300 at the longest wavelengths. There are four channels separated in wavelength by dichroics, with between 12 and 21 image slices. Depending on wavelength, the field of view ranges from 3.70″×3.70″ to 7.74″×7.95″. Each individual exposure provides two wavelength ranges on two detectors; three exposures are required to get a full wavelength spectrum. MIRI MRS is used in many programs; an example is Program 1549 which will observe three molecule-rich protoplanetary disks that were shown to have very bright water line emission in Spitzer spectra.

V.4.4 MIRI coronagraphic imaging

The imaging channel on MIRI includes four coronagraphs for high-contrast imaging. The four coronagraphs are optimized for observations at 10.65μm, 11.40μm, 15.50μm and ∼23μm. The short wavelength coronagraphs use four-quadrant phase masks [116, 115, 4QPM;], while the other is a more traditional Lyot design with an occulting spot in the image plane and a stop in the pupil plane. The 4QPMs are usable at a smaller inner working angle than more traditional designs, and can reach near 1λ/D. Each of the 4QPMs provides a field of 24″×24″, while the Lyot spot mask provides 30″×30″ field of view. Each of the coronagraphs demonstrated a raw contrast ratio >10,000 at 6λ/D during commissioning. An example using this mode is Program 1618, which will search for planets and zodiacal dust around Alpha Centauri A.