How engineers will calibrate James Webb’s four instruments

The James Webb Space Telescope is inching closer to beginning its science mission. With the mirrors and instruments all fully aligned, Webb is undertaking the next big step in getting ready for science operations this summer: Commissioning the instruments. Webb has four instruments, the Near-Infrared Camera (NIRCam), Near-Infrared Spectrometer (NIRSpec), Near-Infrared Imager and Slitless Spectrometer (NIRISS) / Fine Guidance Sensor (FGS), and the Mid-Infrared Instrument (MIRI).

The engineers need to check that every part of each instrument is working exactly as it should so that the telescope can be as accurate as possible. All four instruments have already been powered up and cooled to their operating temperatures. To check their components, the commissioning team operates the instruments’ mechanisms like the wheels which control the filters and gratings, and NIRSpec’s microshutters which are like tiny windows that open and close to allow it to image hundreds of targets at once.

Then it’s on to the instrument calibrations, in which each instrument collects data from a known target and is then tweaked to ensure it is accurate, as Scott Friedman, lead commissioning scientist for Webb, explained in a NASA blog post: “The astrometric calibration of each instrument maps the pixels on the detectors to the precise locations on the sky, to correct the small but unavoidable optical distortions that are present in every optical system. We do this by observing the Webb astrometric field, a small patch of sky in a nearby galaxy, the Large Magellanic Cloud.”

The Large Magellanic Cloud is a well-known astronomical object, which has previously been observed with great accuracy by the Hubble Space Telescope. That means the engineers have a very good comparison for what they should be seeing with Webb. This allows them to calibrate the tiny optical distortions in the instruments and allow for them.

“Calibrating this distortion is required to precisely place the science targets on the instruments’ field of view,” Friedman explained. “For example, to get the spectra of a hundred galaxies simultaneously using the NIRSpec microshutter assembly, the telescope must be pointed so that each galaxy is in the proper shutter, and there are a quarter of a million shutters!” The team will also test out the sharpness of images gathered from each instrument, and test out whether the instruments can correctly point to a given target. The last step is to check whether instruments can track moving targets, which is not needed for most observations because the targets are so far away, but it is useful for looking at targets like asteroids and comets in our solar system.

“We are now in the last two months of Webb’s commissioning before it is fully ready for its scientific mission,” Friedman writes. “We still have important properties and capabilities of the instruments to test, measure, and demonstrate.

When these are complete, we will be ready to begin the great science programs that astronomers and the public alike have been eagerly awaiting.

We are almost there.”