NASA's James Webb Space Telescope

JWST Focal Plane Layout

JWST Focal Plane Layout
For a telescope boresight orientation of 0 degrees (i.e., V1 axis on the celestial equator), the V2 axis points to North.

The JWST instruments view different portions of the JWST focal plane, as shown in the diagram above.  From left to right:

The Near-Infrared Spectrograph

The NIRSpec micro-shutter array (MSA) has four distinct apertures, separated by gaps. The image being used in the illustration shows a set of low-resolution spectra obtained during ground testing. Between the MSA apertures there are three fixed slits for observing individual objects (including a large slit for exoplanet transits). These are shown in red in the diagram. There is also a 3x3 arcsecond integral-field unit (the orange outline), which provides a three-dimensional data cube, with two spatial and one spectral dimension. 

The Fine-Guidance Sensor

The Fine-Guidance Sensor (FGS), shown as greyed out boxes, has two separate imaging arrays, each with fields of view about 2.4 arcminutes on a side. Sofware onboard identifies guide stars from an uploaded catalog. Once acquired, the sensors measure the positions of the stars and feed them to the pointing control system, which adjusts the pointing of the telescope and position of the fast-steering mirror to compensate for any spacecraft motions (or to track moving targets in our own solar system). 

The Near-Infrared Camera

NIRCam is at the center of the diagram. There are two identical modules. The incoming light passes through a dichroic, which splits the bean at about 2.35 μm, reflecting the short wavelengths (0.6-2.3 μm), and transmitting the long wavlengths (2.4-5 μm). The fields-of-view of the short- and long-wavelength channels therefore overlap on the sky, with slight differences shown by the red and blue outlines in the figure above. The long-wavelength channel has one sensor array, while the short-wavelength channel uses four arrays, with gaps between them as shown. NIRCam has a suite of coronagraphic masks that can be placed into the beam for high-contrast imaging. Normally the coronograph is outside the field of view of the detectors, but by inserting an optical wedge into the beam the coronograph is brought into view. There are five occulters in each module, with masks optimized for different wavelengths. Each corongraphic aperture has  a field of view of 20"x20", with a nearby neutral-density square for each one to assist in target acquisition. 

The Near-InfraRed Imager and Slitless Spectrograph

The NIRISS field of view is at the bottom right. Its different observing modes (imaging, slitless spectroscopy, and aperture-mask interferometric imaging) share the same field of view, although may read out only a portion of the detector in some modes. 

The Mid-Infrared Instrument 

MIRI is shown at the top right. The instrument field of view is divided into sections. The largest is the 74"x113" imager for broad-band observations from 5 to 28 μm. Next largest is the 30"x30" Lyot coronograph, which is optimized for observations at 23 μm. For high-contrast imaging, that is accompanied by three four-quadrant phase masks (4QPMs) with effective field sizes of 24"x24". In this diagram, the bottom-most 4QPM is the one optimized for the shortest wavelength (10.65 μm), and the two above it are for 11.4 and 15.5 μm.  The small orange box to the far right outlines the Integral Field Unit spectrometer (IFU). The incoming beam to the is split into four wavelength channels. They thus end observing simultaneously centered at roughly the same spot on the sky. However their fields of view are different, ranging from 3.9" on a side for the shortest wavelength channel to 7.7" for the longest channel. Finally, there is a low-resolution spectroscopic mode, which can observe either through a 0.51"x4.7" slit, or in a slitless configuration. The slit is shown in red between the imager and the 4QPM.