The James Webb Space Telescope’s Four Main Instruments Are Being Commissioned
Following a common theme over the past few months, the James Webb Space Telescope has continued to make impressive progress as it works towards consistent science operations. After an initial launch in late 2021, now in the middle of May, Webb is closer than ever to being complete. This means we are not far away from some incredible images and discoveries of the Universe.
Specifically, NASA has provided a few updates regarding the next generation telescope and its progress. This not only includes another fascinating image recently taken, but also the start of a 17 stage check out for the four main instruments. All of which feature different purposes to ensure Webb is ready to provide one of a kind images and information.
Over the past couple months, NASA has continued to express its satisfaction with how the JWST is performing. It seems the telescope keeps outdoing itself and proving to be more powerful and accurate than expected. These 17 modes that need to be checked are one of the final steps before Webb is scheduled to be complete later this summer. Here I will go more in-depth into the recent image produced, the 4 main instrument checks, and what to expect in the near future.
Most Recent Image
In the past week alone Webb has made significant progress in multiple different areas. One of the first updates included another image highlighting what the JWST is capable of. Specifically, a few days ago on May 9th, NASA tweeted saying, “Computer, enhance! Compare the same target — seen by Spitzer & in Webb’s calibration images. Spitzer, NASA’s first infrared Great Observatory, led the way for Webb’s larger primary mirror & improved detectors to see the infrared sky with even more clarity.” This tweet included two images taken by different telescopes. Here, a close-up of the MIRI image is compared to a past image of the same target taken with NASA’s Spitzer Space Telescope’s Infrared Array Camera (at 8.0 microns). The retired Spitzer telescope was one of NASA’s Great Observatories and the first to provide high-resolution images of the near- and mid-infrared universe. Webb, with its significantly larger primary mirror and improved detectors, will allow us to see the infrared sky with improved clarity, enabling even more discoveries. For example, Webb’s MIRI image shows the interstellar gas in unprecedented detail. Here, you can see the emission from “polycyclic aromatic hydrocarbons,” or molecules of carbon and hydrogen that play an important role in the thermal balance and chemistry of interstellar gas. When Webb is ready to begin science observations, studies such as these with MIRI will help give astronomers new insights into the birth of stars and protoplanetary systems. This is just one of many exciting updates and progress reports coming from the telescope.
Checking The 17 Modes
Not only has Webb taken another promising image, but it has also begun its final commissioning activities. On May 9th, NASA tweeted again saying, “Check please! Before we can #UnfoldTheUniverse & start science, our team must test, calibrate, verify and sign off on each mode of operation for Webb’s 4 instruments. There are 17 instrument modes in total! Follow along on our “Where Is Webb?” tracker.” In other words, with the telescope optics and instruments aligned, the Webb team is now commissioning the observatory’s four powerful science instruments. There are 17 different instrument “modes” to check out on the way to getting ready for the start of science this summer. Once NASA has approved all 17 of these modes, NASA’s James Webb Space Telescope will be ready to begin scientific operations. Each mode has a set of observations and analysis that need to be verified, and it is important to note that the team does not plan to complete them in the exact order shown. Some of the modes won’t be verified until the very end of commissioning. For each mode, NASA has also selected a representative example science target that will be observed in the first year of Webb science. These are just examples; each mode will be used for many targets, and most of Webb’s science targets will be observed with more than one instrument and/or mode. The detailed list of peer-reviewed observations planned for the first year of science with Webb ranges from our solar system to the most distant galaxies.
The 17 modes are separated into 4 main groups including NIRCam with 5 modes, and NIRSpec, NIRISS, and MIRI with 4 modes. The first group is the Near-Infrared Camera, or (NIRCam) imaging. Near-infrared imaging will take pictures in part of the visible to near-infrared light, 0.6 to 5.0 micrometers wavelength. This mode will be used for almost all aspects of Webb science, from deep fields to galaxies, star-forming regions to planets in our own solar system. An example target in a Webb cycle 1 program using this mode: the Hubble Ultra-Deep Field. During the next check, spectroscopy separates the detected light into individual colors. Slitless spectroscopy spreads out the light in the whole instrument field of view so we see the colors of every object visible in the field. Slitless spectroscopy in NIRCam was originally an engineering mode for use in aligning the telescope, but scientists realized that it could be used for science as well. For the third mode, when a star has exoplanets or dust disks in orbit around it, the brightness from a star usually will outshine the light that is reflected or emitted by the much fainter objects around it. Coronagraphy uses a black disk in the instrument to block out the starlight in order to detect the light from its planets. You then have imaging. Most astronomical objects change on timescales that are large compared to human lifetimes, but some things change fast enough for us to see them. Time series observations read out the instruments’ detectors rapidly to watch for those changes. Finally, when an exoplanet crosses the disk of its host star, light from the star can pass through the atmosphere of the planet, allowing scientists to determine the constituents of the atmosphere with this spectroscopic technique. Scientists can also study light that is reflected or emitted from an exoplanet, when an exoplanet passes behind its host star. This completes the list for the 5 NIRCam modes.
Next is the Near-Infrared Spectrograph, or (NIRSpec) multi-object spectroscopy. NIRSpec has a microshutter device with a quarter of a million tiny controllable shutters. Opening a shutter where there is an interesting object and closing the shutters where there is not allows scientists to get clean spectra of up to 100 sources at once. In addition to the microshutter array, NIRSpec also has a few fixed slits that provide the ultimate sensitivity for spectroscopy on individual targets. On the third mode, integral field unit spectroscopy produces a spectrum over every pixel in a small area, instead of a single point, for a total of 900 spatial/spectral elements. This mode gives the most complete data on an individual target. Lastly, NIRSpec can obtain a time series spectroscopic observation of transiting exoplanets and other objects that change rapidly with time. These are the 4 modes and checks currently in progress prior to the completion of Webb.
The third test group includes the Near-Infrared Imager and Slitless Spectrograph, (NIRISS) single object slitless spectroscopy. In order to observe planets around some of the brightest nearby stars, NIRISS takes the star out of focus and spreads the light over lots of pixels to avoid saturating the detectors. In the next check NIRISS includes a slitless spectroscopy mode optimized for finding and studying distant galaxies. This mode will be especially valuable for discovery, finding things that we didn’t already know were there. The third mode includes NIRISS aperture masking interferometry. NIRISS has a mask to block out the light from 11 of the 18 primary mirror segments in a process called aperture masking interferometry. This provides high-contrast imaging, where faint sources next to bright sources can be seen and resolved for images. For the final mode, due to the importance of near-infrared imaging, NIRISS has an imaging capability that functions as a backup to NIRCam imaging.
Finally, there are 4 modes for the Mid-Infrared Instrument, (MIRI) imaging. Just as near-infrared imaging with NIRCam will be used on almost all types of Webb targets, MIRI imaging will extend Webb’s pictures from 5 to 27 microns, the mid-infrared wavelengths. Mid-infrared imaging will show us, for example, the distributions of dust and cold gas in star-forming regions in our own Milky Way galaxy and in other galaxies. The other three modes include low-resolution spectroscopy, medium-resolution spectroscopy, and coronagraphic imaging. Altogether there are 17 modes that need to be checked during Webb’s final commissioning activities.
Conclusion
For almost half a year now, the James Webb Space Telescope has been in space working towards consistent science operations. Just recently it not only provided more high-quality images but also began working on its final commissioning activities. This includes 17 modes that need to be checked for the 4 main instruments onboard the telescope. We will have to wait and see how it progresses and the impact it has on the space industry.
