NASA's James Webb Space Telescope is now experiencing all seasons—from hot
to cold—as it undergoes the thermal stability test. Meanwhile, activities
are underway for the final phase of commissioning: digging into the details
of the science instruments, the heart of Webb. To complete commissioning, we
will measure the detailed performance of the science instruments before we
start routine science operations in the summer.
Today, the lead commissioning scientist for Webb, 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 near its final
cryogenic temperature, we are ready to begin the last group of activities
before the science observations start: science instrument commissioning.
Here I describe just 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 safely cooled. We have operated their mechanisms
and detectors, including filter wheels, grating wheels, and the NIRSpec
microshutter assembly. The Webb optics team used images of isolated stars
taken with each of the instruments to align the primary and secondary
mirrors of the observatory. But we have more work to do before Webb is fully
ready to embark on the ambitious science observations that will reveal the
secrets of the universe.
"We will now begin an extensive suite of calibrations and characterizations
of the instruments using a rich variety of astronomical sources. We will
measure the instruments' throughput—how much of the light that enters the
telescope reaches the detectors and is 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 arrives. We will
measure this throughput at multiple 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 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. This field was observed by the
Hubble Space Telescope to establish the coordinates of about 200,000 stars
to an accuracy of 1 milli-arcsec (less than 0.3 millionths of a degree).
Calibrating this distortion is required to precisely place the science
targets on the instruments' field of view. 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!
"We will also measure the sharpness of the stellar images, what astronomers
call the 'point spread function.' We already know the telescope is
delivering to the instruments image quality that exceeds our prelaunch
expectations, but each instrument has additional optics. These optics
perform a function, such as passing the light through filters to get color
information about the astronomical target or using a diffraction grating to
spread the incoming light into its constituent colors. Measuring the point
spread function within each instrument at different wavelengths provides an
important calibration for interpreting the data.
"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 science target
relative to that guide star. This places the science target to an accuracy
of a few tenths of an arcsecond. However, in some cases more precision is
necessary, approximately a hundredth of an arcsecond. For example, for
coronagraphy, the star has to be placed behind a mask so its light is
blocked, allowing the nearby exoplanet to shine through. In time series
observations, we measure how an exoplanet's atmosphere absorbs the stellar
light during the hours it takes to pass in front of its star, allowing us to
measure the properties and constituents of the planet's atmosphere. Both of
these applications require that the instrument send corrections to the
telescope pointing control system to put the science target precisely in the
correct location within the instrument's field of view.
"A final example of our instrument commissioning activities is observations
of moving targets. Most astronomical objects are so far away that they
appear to be stationary on the sky. However, this is not true of the
planets, satellites and rings, asteroids, and comets within our own solar
system. Observing these requires that the observatory change its pointing
direction relative to the background guide stars during the observation. We
will test this capability by observing asteroids of different apparent
speeds using each instrument.
"We are now in the last two months of Webb's commissioning before it is
fully ready for its scientific mission. 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."
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Space & Astrophysics