This paper describes and tests a qualification setup for the primary mirror and underlying structure of the James Webb Space Telescope (JWST). This primary mirror has a diameter of 6.5m and consists of 18 segments. The qualification setup verifies whether the segments and structure can resist the forces that occur during the launch of the telescope. This is done by measuring a segment, doing a launch test by exposing the segment to forces similar to those occurring during launch, and measuring it again to detect changes. These changes should not exceed a few tens of nanometers.
Although statically measuring nanometers is delicate, it is daily practice. However, challenges for qualifying the JWST are of a different proportion. The size of the optics is 1.3m; although measuring perfect spherical mirrors is relatively easy, the optics of the JWST all have off-axis aspherical shapes; and deformations of the telescope structure are not necessarily visible on the surface of the mirror segments.
The interesting aspect of the qualification setup presented here is that it meets these challenges by means of a customized interferometer that uses the newest technologies available. Computer-generated holograms enable measuring off-axis aspherical shapes by nulling the asphericity of the mirrors. A pixel-wise phase-shifting element in combination with a high speed camera is used to enable dynamic measurements at a frame rate of up to kHz. With these measurements the changes in dynamics can be measured, which is an indicator for structural deformations. These measurements do not focus on the absolute shape of the mirrors, but rather on the changes in shape and the dynamics that can be caused by the launch test.
Although these measures solve the fundamental measurement issues, the lab-environment also gives rise to wavefront aberrations that distort the measurements: different alignment before and after the launch tests; the surface shapes of the used optical components are not perfect; the set-up was close to a cleanroom air outlet causing turbulence and stratification. The Beryllium mirrors, which are both light and stiff, have a low expansion coefficient around 50 K but not at room temperature, and this gives rise to thermal deformations of multiple nms caused by thermal gradients. Errors of this kind were minimized by a dedicated alignment procedure to minimize aberrations and by stabilizing the room temperature to less than 0.05K for 10 minutes. With these measures, the test setup was able to reach a reproducibility of several tens of nanometers. In addition, the authors used a statistical metric to quantify the differences in the dynamic tests. Despite the fact work still needs to be done, the authors show that this qualification of the primary JWST mirror is within reach. These qualifications give confidence that the primary mirror of the JWST will survive launch and can start looking at the origins of the universe.
From my perspective, the authors show more than only a qualification tool for the JWST. They also provide a glimpse of the future of high precision surface metrology: measurements of aspherical components, high speed interferometry for thermal sensitive components.
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