The final phase of commissioning

Artist’s impression of NASA’s James Webb Space Telescope. Credit: NASA

NASA’s James Webb Space Telescope now experiences all seasons, from hot to cold, as it undergoes the thermal stability test. Meanwhile, activities are underway for the final phase of commissioning: delving into the details of the science instruments, Webb’s heart. To complete the commissioning, we will measure the detailed performance of the science instruments before beginning routine science operations over the summer.

Today, Webb’s commissioning scientist, Scott Friedman of the Space Telescope Science Institute (STScI), gives us all the details on this final phase of commissioning.

“With the telescope beautifully aligned and the observatory close to its final cryogenic temperature, we are ready to begin the last set of activities before science observations begin: commissioning the science instruments. one of these activities.

“The instruments, Near Infrared Camera (NIRCam), Near Infrared Spectrometer (NIRSpec), Near Infrared Imager and Slitless Spectrometer (NIRISS), Mid Infrared Instrument (MIRI ) and Fine Guidance Sensor (FGS) were safely powered and cooled, and we ran their mechanisms and detectors, including filter wheels, grid wheels, and the NIRSpec micro-shutter assembly. The Webb optics team used images of single stars taken with each of the instruments to align the primary and secondary mirrors of But we still have work to do before Webb is fully ready to embark on the ambitious scientific observations. that will reveal the secrets of the universe.

“We will now begin an extensive series of instrument calibrations and characterizations using a rich variety of astronomical sources. We will measure instrument throughput – the amount of light that enters the telescope reaches the detectors and is registered. There is always some loss with each reflection by the mirrors of the telescope and inside each instrument, and no detector registers each photon that arrives. We will measure this rate at several wavelengths of light by observing standard stars whose light emission is known from data obtained with other observatories combined with theoretical calculations.

“The astrometric calibration of each instrument maps detector pixels to precise locations in the sky, to correct for the small but unavoidable optical distortions present in each optical system. We do this by observing the Webb astrometric field, a small patch of sky in a nearby galaxy, the Large Magellanic Cloud.This field has been observed by the Hubble Space Telescope to establish the coordinates of about 200,000 stars with an accuracy of 1 milli-arcsec (less than 0.3 millionths of a degree). necessary to accurately place scientific targets on the field of view of the instruments. For example, to obtain the spectra of a hundred galaxies simultaneously using the NIRSpec micro-shutter assembly, the telescope must be pointed from so that every galaxy is in the correct shutter, and there are a quarter of a million shutters!

“We will also measure the sharpness of stellar images, what astronomers call the ‘point spread function.’ We already know that the telescope provides the instruments with image quality that exceeds our pre-launch expectations, but each instrument has additional optics.These optics perform a function, such as passing light through filters to obtain color information about the astronomical target or using grating diffraction to split the incoming light into its constituent colors. Point spread function in each instrument at different wavelengths provides important calibration for data interpretation.

“We will test target acquisition for each instrument. For some observations, it is sufficient to point the telescope using the position of a guide star in the fine guidance sensor and know the location of the scientific target relative to this guide star. This places the science aims for an accuracy of a few tenths of an arc second. However, in some cases greater accuracy is needed, around a hundredth of an arc second. For example, for coronagraphy, the star must be placed behind a mask so that its light is blocked, allowing the nearby exoplanet to shine in. In time-series observations, we measure how an exoplanet’s atmosphere absorbs starlight during the hours it ‘it puts to pass in front of its star, which allows us to measure the properties and constituents of the planet’s atmosphere.Both of these applications require the instrument to send correct ions to the telescope’s pointing control system to place the scientific target precisely in the right place in the instrument. the field of view.

“A final example of our instrument commissioning activities is the observation of moving targets. Most astronomical objects are so far away that they appear stationary in the sky. However, planets are not. , satellites and rings, asteroids and comets in our own solar system. Observing them requires the observatory to change its pointing direction relative to the background guide stars during the observation. We will test this capability by observing asteroids of different apparent velocities using each instrument.

“We are now in the final two months of Webb’s commissioning before it is fully ready for its science mission. We still have significant instrument properties and capabilities to test, measure and demonstrate. When they completed, we will be ready to begin the major science programs that astronomers and the public have been eagerly awaiting.

NASA’s Webb Telescope is now in focus, ready for instrument commissioning

Quote: Examining the heart of Webb: The final phase of commissioning (2022, May 6) retrieved May 6, 2022 from

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