Light Waves Used to Align Main Mirror Segments of NASA's James Webb Space Telescope

A team of Engineers at NASA’s Johnson Space Center in Houston made use of light waves to align the James Webb Space Telescope’s mirror segments to each other, so they behave like a single, monolithic mirror in the cryogenic cold of the center’s iconic Chamber A.

Part of the Webb telescope’s continuing cryogenic testing in Chamber A at Johnson includes aligning, or “phasing,” the telescope’s 18 hexagonally-shaped primary mirror segments so they operate as a single 6.5 m mirror. All of these segments must have the precise position and precise curvature; or else, the telescope will fail to accurately focus on its celestial targets.

Aligning the mirrors

To measure the shape of the Webb telescope’s main mirror, the Engineers use a test device known as an interferometer, which shines a laser down onto the mirror. Because the mirror is segmented, it requires a specifically designed interferometer, known as a multi-wavelength interferometer, which allows the Engineers to use two light waves at the same time, explained Lee Feinberg, Optical Telescope Element Manager for the Webb telescope at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The interferometer splits the laser light into two single waves. One of these waves goes via a lens and reflects off the main mirror; the other wave acts as a reference. The reflected wave interferes with (meets) the reference wave, and the Engineers examine the combined wave that results from that interference.

By analyzing the interference signal, engineers determine the mirror shape and the alignment of the mirrors.

Lee Feinberg, Optical Telescope Element Manager, the Webb telescope, NASA’s Goddard Space Flight Center, Greenbelt, Maryland

When the Engineers need to tweak the shapes and positions of the mirror segments to attain precise alignment, they use the seven actuators (miniature mechanical motors) attached to the back of each one of the mirror segments. For each segment, six of these actuators are positioned into groups of two, at three equally spaced points along the outer side of the mirror (to tweak the segment’s position), and one is attached to six struts that are connected to each of the hexagonal mirror segment’s corners (to tweak the segment’s shape).

The actuators on each mirror segment are capable of very minute movements, which allow Engineers to align the complete main mirror by finely tweaking each mirror segment.

They can move in steps that are a fraction of a wavelength of light, or about 1/10,000^th the diameter of a human hair.

Lee Feinberg, Optical Telescope Element Manager, the Webb telescope, NASA’s Goddard Space Flight Center, Greenbelt, Maryland

These actuators can also be employed to precisely reshape each mirror segment to guarantee they all match up once aligned. The ability to alter the mirror alignment and shape is crucial because the mirror must be opened from its unaligned stowed position when the telescope is set up. This test confirms the actuators have adequate range of movement once they are in space, at their operational temperature of around 40 K (or about -388 °F / -233 °C), to put the telescope’s main mirror into its precise shape so it can accurately investigate the universe.

Testing the aligned mirrors

Once the mirrors are aligned, the Engineers test Webb’s optics using a support equipment known as the ASPA (a nested acronym that means “AOS Source Plate Assembly). The ASPA is a piece of test hardware that is placed on top of Webb’s Aft Optics Subsystem (AOS) and transmits test laser light into and out of the telescope, thereby acting like a source of artificial starlight. The AOS comprises of the telescope’s tertiary and fine-steering mirrors.

During one portion of the optical test, known as the “half-pass” test, the ASPA feeds laser light straight into the AOS, where it is directed by the tertiary and fine-steering mirrors to Webb’s four science instruments, which lie in a compartment directly behind the main mirror. This test allows the Engineers to take measurements of the optics inside the AOS to confirm that Webb’s tertiary mirror, which is immovable, is properly aligned to the instruments.

In another portion of the test, known as the “pass-and-a-half” test, light travels in a reverse path via the telescope optics. The light is again fed into the system from the ASPA, but upwards this time, to the secondary mirror. The secondary mirror reflects the light down to the main mirror, which transmits it back up to the top of Chamber A. Mirrors at the top of the chamber transmit the light back down to the telescope again, where it follows its regular course through the telescope to the instruments, but this time avoiding the ASPA test equipment.

This verifies not only the alignment of the primary mirror itself but also the alignment of the whole telescope — the primary mirror, secondary mirror, and the tertiary and fine-steering mirrors inside the AOS. Taken together, the half-pass and pass-and-a-half tests demonstrate that everything is aligned to everything else.

Paul Geithner, Deputy Project Manager, the Webb telescope, NASA’s Goddard Space Flight Center, Greenbelt, Maryland

As the ASPA is ground test hardware, it will be removed from the telescope after the cryogenic testing at Johnson is done.

The cryogenic vacuum environment of Chamber A mimics the frigid space environment where Webb will function, and where it will gather data of never-before-seen portions of the universe. Confirming the whole telescope, including its optics and instruments, works correctly in this cold environment ensures the telescope will function correctly in space. The telescope and its instruments are engineered to operate cold, so they must be cold to be aligned and to correctly function.

The James Webb Space Telescope is the scientific complement to NASA's Hubble Space Telescope. It will be the most powerful space telescope ever constructed. Webb is an international project led by NASA with its partners, European Space Agency (ESA) and the Canadian Space Agency (CSA).

Engineers at NASA’s Johnson Space Center in Houston used light waves to align the James Webb Space Telescope’s mirror segments to each other, so they act like a single, monolithic mirror in the cryogenic cold of the center’s iconic Chamber A. (Credits: NASA's Goddard Space Flight Center/Mike McClare)