NASAs James Webb Space Telescope. Credit: NASA Goddard Space Flight Center and Northrup Grumman
NASAs James Webb Space Telescope is now experiencing all seasons– from hot to cold– as it undergoes the thermal stability test. On the other hand, activities are underway for the last stage of commissioning: diving into the information of the science instruments, the heart of Webb. To complete commissioning, we will measure the comprehensive efficiency of the science instruments before we begin routine science operations in the summertime.
Today, the lead commissioning researcher for Webb, Scott Friedman of the Space Telescope Science Institute (STScI), gives us all the details on this last phase of commissioning.
” With the telescope beautifully aligned and the observatory near its final cryogenic temperature, we are prepared to begin the last group of activities before the science observations start: science instrument commissioning. Here I describe simply a few of those activities.
” The instruments, the Near-Infrared Camera (NIRCam), Near-Infrared Spectrometer (NIRSpec), Near-Infrared Imager and Slitless Spectrometer (NIRISS), Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor (FGS) have been powered up and securely cooled. The Webb optics team used images of separated stars taken with each of the instruments to line up the main and secondary mirrors of the observatory.
We will measure the instruments throughput– how much of the light that goes into the telescope reaches the detectors and is tape-recorded. There is always some loss with each reflection by the mirrors of the telescope and within each instrument, and no detector records every photon that gets here.
The beam of light coming from the telescope is then shown in deep blue entering the instrument through the pick-off mirror situated at the top of the instrument and acting like a periscope.Then, a series of mirrors reroute the light toward the bottom of the instruments where a set of 4 spectroscopic modules are situated. Each beam enters its own important field unit; these elements split and reformat the light from the whole field of view, ready to be distributed into spectra. This needs the light to be folded, bounced, and split lots of times, making this probably one of Webbs most complicated light paths.To finish this amazing trip, the light of each beam is distributed by gratings, creating spectra that then forecasts on 2 MIRI detectors (2 beams per detector).
” The astrometric calibration of each instrument maps the pixels on the detectors to the accurate areas on the sky, to fix the little however unavoidable optical distortions that are present in every optical system. Calibrating this distortion is needed to precisely position the science targets on the instruments field of view.
We currently understand the telescope is providing to the instruments image quality that exceeds our prelaunch expectations, but each instrument has additional optics. Determining the point spread function within each instrument at various wavelengths provides an important calibration for analyzing the information.
” We will evaluate target acquisition for each instrument. For some observations, it is enough to point the telescope using the position of a guide star in the Fine Guidance Sensor and understand the location of the science target relative to that guide star. Both of these applications need that the instrument send corrections to the telescope pointing control system to put the science target exactly in the correct area within the instruments field of view.
” A last example of our instrument commissioning activities is observations of moving targets. Many astronomical things are so far away that they seem fixed on the sky. This is not true of the planets, satellites and rings, asteroids, and comets within our own solar system. Observing these needs that the observatory change its pointing instructions relative to the background guide stars throughout the observation. We will evaluate this capability by observing asteroids of various apparent speeds utilizing each instrument.
” We are now in the last two months of Webbs commissioning prior to it is completely all set for its clinical objective. We still have important properties and abilities of the instruments to test, measure, and demonstrate.
— Scott Friedman, lead commissioning scientist for Webb, STScI
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” The instruments, the Near-Infrared Camera (NIRCam), Near-Infrared Spectrometer (NIRSpec), Near-Infrared Imager and Slitless Spectrometer (NIRISS), Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor (FGS) have been powered up and safely cooled. We will determine the instruments throughput– how much of the light that gets in the telescope reaches the detectors and is tape-recorded. The beam of light coming from the telescope is then revealed in deep blue getting in the instrument through the pick-off mirror situated at the top of the instrument and acting like a periscope.Then, a series of mirrors reroute the light towards the bottom of the instruments where a set of 4 spectroscopic modules are located. We currently understand the telescope is providing to the instruments image quality that surpasses our prelaunch expectations, but each instrument has extra optics. Both of these applications require that the instrument send out corrections to the telescope pointing control system to put the science target exactly in the proper place within the instruments field of view.
Jonathan Gardner, Webb deputy senior job researcher, NASA Goddard
Stefanie Milam, Webb deputy task researcher for planetary science, NASA Goddard