Skip to main content

How engineers will calibrate James Webb’s 4 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.”

Editors' Recommendations

Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
Scientists explain cosmic ‘question mark’ spotted by Webb space telescope
The shape of a question mark captured by the James Webb Space Telescope.

Considering the myriad of unknowns that still exist for scientists exploring the vastness of the universe, the recent discovery in deep space of what seems to be a giant question mark feels highly appropriate.

Captured by the powerful James Webb Space Telescope, the bright, distinctive object clearly bears the shape of a question mark, leaving some stargazers wondering if the cosmos is teasing us, or perhaps motivating us to keep on searching the depths of space for the secrets that it may reveal.

Read more
James Webb telescope captures the gorgeous Ring Nebula in stunning detail
JWST/NIRcam composite image of the Ring Nebula. The images clearly show the main ring, surrounded by a faint halo and with many delicate structures. The interior of the ring is filled with hot gas. The star which ejected all this material is visible at the very centre. It is extremely hot, with a temperature in excess of 100,000 degrees. The nebula was ejected only about 4000 years ago. Technical details: The image was obtained with JWST's NIRCam instrument on August 4, 2022. Images in three different filters were combined to create this composite image: F212N (blue); F300M (green); and F335M (red).

A new image from the James Webb Space Telescope shows the stunning and distinctive Ring Nebula -- a gorgeous structure of dust and gas located in the constellation of Lyra. This nebula is a favorite among sky watchers as it faces toward Earth so we can see its beautiful structure, and because it is visible throughout the summer from the Northern Hemisphere. It is different from the Southern Ring nebula, which Webb has also imaged, but both are a type of object called a planetary nebula.

Located just 2,600 light-years away, the Ring Nebula is a structure of dust and gas that was first observed in the 1770s, when it was thought to be something like a planet. With advances in technology, astronomers realized it was not a planet, but rather a cloud of dust and gas, and thanks to highly detailed observations by space telescopes like Hubble and Webb, scientists have been able to see more of its complex structure. The nebula isn't a simple sphere or blob, but is rather a central, football-shaped structure surrounded by rings of different material.

Read more
See how James Webb instruments work together to create stunning views of space
The irregular galaxy NGC 6822.

A series of new images from the James Webb Space Telescope shows the dusty, irregular galaxy NGC 6822 -- and the different views captured by various Webb instruments.

Located relatively close by at 1.5 million light-years from Earth, this galaxy is notable for its low metallicity. Confusingly, when astronomers say metallicity they do not mean the amount of metals present in a galaxy, but rather the amount of all heavy elements -- i.e., everything which isn't hydrogen or helium. This factor is important because the very earliest galaxies in the universe were made up almost entirely of hydrogen and helium, meaning they had low metallicity, and the heavier elements were created over time in the heart of stars and were then distributed through the universe when some of those stars went supernova.

Read more