Lessons learned from testing the James Webb Space Telescope optical components
In-process optical testing and final compliance testing of the James Webb Space Telescope (JWST) Optical Telescope Element (OTE) optical components is probably the most difficult astronomical optics metrology job of our generation. The JWST OTE is a three-mirror anastigmatic (TMA) telescope with a primary, secondary, tertiary, and fine-steering mirror. Because of mass and thermal stability considerations, all components are beryllium.1 The primary and secondary mirrors are the most difficult to test. The 6.5m parabolic primary consists of 18 1.5m point-to-point hexagonal segments with three unique prescriptions all of which must have identical 16m radii of curvature (to a tolerance of ±0.10mm) and a surface figure of better than 24nm rms, all at a temperature of 30K. The secondary mirror is a 0.75m-diameter convex hyperboloid whose prescription and <24nm rms surface figure must be certified at 20K.2
The optical testing challenges are multiple. Besides the size of the optical components, there are issues of how to ensure that components manufactured at ambient temperature satisfy their requirements at cryogenic ones to the required tolerances. This is accomplished by measuring the mirrors at 30K and correcting them in ambient conditions.
The first problem encountered by our test team was vibration. When testing a mirror with a 16m radius of curvature, it is easy to have relative motion between the mirror and interferometer, particularly when the mirror is inside a cryo-vacuum chamber. At the NASA Marshall Space Flight Center (MSFC) X-Ray and Cryogenic test Facility (XRCF), while the test optic and the interferometer are each isolated from the building, they are not physically connected to each other. Thus, they experienced relative motion of 5–10μrad (microradians) of tilt and 5–15μm of piston. This magnitude of motion made it virtually impossible to acquire data using conventional temporal phase-shifting interferometry. The solution was found in a breadboard concept at Metrolaser that, after NASA MSFC development funding, yielded the first ever PhaseCAM and has resulted in an entire product line of 4D PhaseCam interferometer products. These interferometers have been a fundamental key technology enabling the manufacture of JWST.
The second problem was how to accurately measure radius of curvature to a precision of 10μm over 16m at 30K. For small optics, radius is typically measured by either an inside micrometer or a distance-measuring interferometer (DMI). But in this case, neither option is viable. It is not possible to insert a calibrated mechanical ‘meter’ into a 30K environment. And, with regard to a DMI, these devices are not absolute. They measure relative distance change, i.e., the motion of the mirror or the interferometer or a cat's eye reflector from the mirror vertex to the mirror center of curvature. Moreover, they require an uninterrupted beam. None of these are possible when testing at 30K into a cryo-vacuum chamber through an optical window. The solution was an absolute distance meter developed by Leica with NASA MSFC funding.
The third problem was thermal stability. To achieve a 20nm rms class mirror requires that the mirror shape is thermally stable to better than 5nm. While beryllium has a very low coefficient of thermal expansion (CTE) below 90K, it has a large CTE at 25°C. And while beryllium is a metal with high thermal conductivity, a highly light-weighted mirror (such as JWST) lacks sufficient thermal capacity to maintain a uniform constant temperature under ambient conditions. Therefore, it is very easy for small thermal gradients to cause significant surface figure errors. To achieve 10nm rms metrology requires that thermal gradients in the mirrors be kept at the 0.01K level. In this case, the solution was to apply proven precision metrology principles: test in an extremely stable thermal environment and monitor the mirror's bulk temperature and gradients.
JWST metrology is performed through a set of guiding principles. First, every step of the manufacturing process must have metrology feedback, and there must be overlap between the metrology tools for a verifiable transition. Second, test accuracy and reproducibility (not repeatability) of the metrology tools must be predicted by error budget, certified by absolute calibration, and verified by independent test. In addition, the test tool uncertainty is formally included in the JWST observatory optical components error budget. Third, ‘Test like you fly’: all components must be certified at 30K. And compliance with all final specifications must be confirmed through an independent cross-check test. Finally, and most important, all anomalies must be fully understood.
In accordance with the first rule, coordinate measuring machines (CMMs) were used for early-stage processes at Axsys Technologies and Tinsley Laboratories Inc. For the secondary mirror and tertiary mirror, it was possible to transition directly from CMM to interferometry. But for the primary mirror segment assemblies (PMSAs), there was a gap. This gap was filled via an IR scanning Shack-Hartmann sensor. Also, full-aperture interferometry did not have sufficient resolution to characterize the mid-spatial frequency requirements or the edge requirements. Therefore, we used a high-spatial frequency (HS) interferometer system.
Regarding the second rule, repeatability is the ability to get the same answer twice if nothing in the test setup is changed. Reproducibility is the ability to get the same answer twice if the mirror is completely removed from and then reinstalled into the test setup. From a real-world manufacturing perspective, reproducibility is much more important than repeatability. This is because on JWST, optical components are not only moved back and forth between manufacturing and test at Tinsley, but also are moved between Tinsley and Ball Aerospace & Technologies Corp. (BATC) and the MSFC XRCF. On JWST, a complete understanding of each metrology tool's test uncertainty is critical. Data from Tinsley, BATC, and the MSFC XRCF must reproduce each other within the test uncertainty. Certified cryo-data must be traceable from XRCF (or BATC) at 30K in the flight mount to BATC at 300K as the mirrors are changed into their fabrication mount to Tinsley where they are polished at 300K.
In addition, every optical component specification test had a confirming test. For the primary mirror segments, the confirming test was a comparison between the computer-generated hologram (CGH) center-of-curvature (CoC) test and an auto-collimation test. For the secondary mirror, the confirming test was a comparison between a Hindle shell test at Tinsley and an aspheric test plate test at BATC. For the tertiary mirror, the confirming test was a comparison between a CoC CGH test and a finite conjugate test.
As specified by the third rule, all components are certified at cryogenic temperatures. PMSA cryo-certification occurs at the MSFC XRCF (see Figure 1). Cryo-certification takes place at BATC for the secondary mirror assembly, tertiary mirror assembly, and fine-steering mirror. Confirmation of the cryo-specifications happens during the observatory level at NASA's Johnson Space Center. Additional cross-check tests are provided by ambient testing at Tinsley, BATC, and XRCF. Figure 2 summarizes the PMSA cryo-requirements, certification test, and cross-check test.
Of all the metrology rules, the fourth is perhaps the most important and must be followed with rigor. No matter how small the anomaly, one must resist the temptation to sweep a discrepancy under the metaphorical error-budget rug. An important example of adherence to this rule is the clear aperture specification. There was a significant discrepancy between the clear aperture measured by the CoC interferometer and the clear aperture measured by the HS interferometer (see Figure 3). The CoC interferometer was measuring a ‘good’ edge while the HS interferometer was measuring a ‘significantly’ down edge. Obviously, only one of these could be right, and using the wrong data to ‘work’ the mirror would result in, at best, a poor convergence rate and, at worst, a mirror that fails to meet spec. The clear aperture has added importance given that it is a key factor in the on-orbit performance. As much as optimists would have liked for the CoC test to be correct, it was not. The HS interferometer was correct, and the fabrication process really was producing a rolled edge. Once the HS data was used to control the process, convergence improved and the mirror clear aperture met the required specification.
The sources of the edge discrepancy are interesting and important to optical metrologists. An early candidate for the discrepancy, but ultimately a nonfactor, is the fact that the test image when viewed through the CGH is distorted. The real source is depth of focus. The aspheric departure of the PMSA is so great that it is not possible to simultaneously have the center and the edge of the mirror in focus. Fresnel diffraction from an out-of-focus edge was coherently adding with the reflected wavefront to obscure the true shape of the PMSA surface at the edge of the mirror. Interestingly, gravity sag also played a role. Astigmatic bending was causing the mirror to be ‘flatter’ in one direction than the other, and thus more in focus in one direction than the other.
In conclusion, JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification, and validation is probably the most difficult astronomical optics metrology job of our generation. But the challenge has been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test setups; several new inventions, including the 4D PhaseCam and Leica absolute distance meter; and strict adherence to a set of four guiding-principle metrology rules. All JWST optical components are meeting their requirements (see Figure 4) and are on schedule. The next step is at the system level in assembly, integration, and testing. Ambient tests will be conducted at Goddard Space Flight Center, and cryogenic system testing will be performed in Chamber A at the Johnson Space Center.
H. Philip Stahl is senior optical physicist at NASA MSFC. He is the JWST mirror optics lead for primary, secondary, and tertiary mirrors. He is a leading authority in optical metrology, engineering, and interferometry. Many of the world's largest telescopes have been fabricated with the aid of interferometers developed by him. He is a SPIE Fellow, past SPIE Director, and current SPIE Vice President to the International Commission on Optics. He chaired the Optical Manufacturing and Testing conference from 1995 to 2005, is the current Optical Manufacturing Track Chair, taught short courses on aspheric and optical testing, and served on SPIE's Engineering, Science, and Technology Policy committee, as well as Nominating, Publication, and Symposia committees.
