How the James Webb Space Telescope works despite getting hot and cold

For sensitive instruments, the sun shield isn't always enough.
Brad Bergan
Webb in space (left), and an artistic rendering of a fiery hot star (right).1, 2

On paper, Webb is a fine telescope. The best.

But we won't know it works until we test it in the extremes of space.

This is why, since the James Webb Space Telescope completed its optical alignment, NASA has entered the final stage of commissioning the space-based observatory's science instruments. Arguably the most unique aspect of Webb at first glance is its massive sun shield, which enables the four primary telescopes aboard to view the universe with less interference from the Sun's light.

But it also serves as a shield from the heat of sunlight, which is why an upcoming thermal stability exercise will measure the way changes "from the hot to cold attitude" affect the space telescope. This will help scientists assess whether it's truly ready to explore the universe as its own little nook of the cosmos around the second Lagrange point (L2) causes subtle changes in its instruments, according to a post on NASA's official website.

Webb is nearly ready to begin its science missions, but the journey of readying the most advanced space telescope ever launched is creating worlds within worlds of engineering marvels, all being enacted in real-time.

Putting the James Webb Space Telescope's engineering to the test

The final stage of Webb's preparation will see all modes and operations of the four onboard science instruments tested on calibration, performance, and general capabilities to observe the mysteries of the universe. As of writing, the James Webb Space Telescope's mirrors are continuing their cooling process, but the forthcoming stability test is crucial.

"Webb’s five-layer sunshield keeps the telescope and science instruments cool and shielded from the Sun, Earth and moon," said Webb's Deputy Observatory Project Scientist Erin Smith, in the blog post from the agency. "This protection allows Webb to make measurements of the infrared universe, which requires a cold telescope and cold instrument optics"

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"However, as Webb points to different targets around the sky, the angle of the Sun on the sunshade changes, which changes the thermal profile of the observatory," continued Smith. "These variations in temperature can induce small changes in the observatory, and affect Webb’s optical quality, pointing, observed backgrounds, and other parameters."

But why, if Webb was built correctly, is the thermal stability exercise necessary? Put simply, it's to test the engineering (it's a practical field, not a theoretical one). As the James Webb Telescope moves from its hot to a cold attitude, it'll spend "multiple days in the cold attitude, then [slew] back to the hot attitude," continued Smith.

Slew 1
Webb in its 'hot' attitude (top), and Webb at its 'cold' attitude (bottom). Source: NASA / STScI

Testing Webb for abnormalities in shifting temperatures

"During this time, the Webb team will measure the thermal stability, pointing performance and optical wavefront drift," added Smith. "In addition to measuring the performance of the observatory, the team will also check the thermal modeling used to predict observatory behavior."

While Webb is in a position that's shielded from sunlight, it'll have access to an annulus (donut-shaped) view of the sky, called the "field of regard". As an entire year passes, the annulus will trace the entire sky.

"Pitch is the angle towards (negative) or away (positive) from the Sun. Webb points between pitches of -5 and +45 degrees," explained Smith in the blog post. "The 'hot' attitude is at 0 degrees, with the Sun squarely illuminating the sunshield. The 'cold' attitude is +45 degrees, with the sunlight reduced by a factor of cosine(45 degrees), about 0.7."

The test itself will place Webb in the hot attitude "at about 0 degrees pitch," where it will remain for five days as thermal stabilization takes its course, said Smith. "The team will make baseline measurements of the pointing stability, optical wavefront error and any oscillations caused by the instrument electronics."

James Webb Space Telescope's 'real-world calibration'

"Once this baseline has been established, the team will slew the observatory to the cold attitude, about +40 degrees pitch," added Smith. "Immediately after the slew, the team will use NIRCam’s suite of weak lenses for 24 hours to continuously measure any short-timescale effects on the wavefront. After this, the team will monitor the stability of the telescope every 12 hours, to measure the thermal stabilization of the telescope itself."

In the hot attitude, the Earth-based team supervising Webb will gather "high-cadence pointing stability data" that employs the NIRCam and the FGS/NIRISS instruments. The infrared instrument designed to observe the extremely young universe, MIRI, will also observe the universe at hot and cold attitudes — just to check that these extremes don't have unforeseen effects on the telescope's ability to view the universe at mid-infrared background levels.

"When assembled together, the data from the thermal stability tests will allow the observatory team to better understand how the observatory behaves thermally," said Smith in the NASA blog post. The temperature changes will be very subtle, but the James Webb Space Telescope is an extremely sensitive piece of equipment — which means subtle environmental variations could drastically affect its performance. Effectively, the test will work as a real-world calibration of the highly complex thermal models used to build Webb's hardware and software. This will ultimately maximize the power of Webb's next-gen technology, making the final release of its first science data that much sweeter.

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