Development and validation of a pneumatic metrology mount for spaceborne mirrors

In the realization of large-size mirrors for high-accuracy space telescopes, the metrology mount, used during interferometric measurement of a mirror's surface figure, is a crucial system in the final surface figure correction stage of mirror fabrication. During these measurements, conveniently, mirrors are placed horizontally (with optical axis vertical), and the purpose of the metrology mount is to enable surface figure measurement in space-like zero-g conditions as the mirror has its operation in space. Considering the advantages of an active metrology mount over proven passive metrology mounts, the development of a pneumatic-based active metrology mount is taken up. A pneumatic metrology mount provides the required multipoint support forces to the mirror at individual support locations through pneumatic actuators by precise and simultaneous control of air pressure regulators using a closed-loop feedback control. In regard to this development, first, sensitivity analysis is carried out to understand the effect of variation in support forces on repeatability of interferometric surface figure measurements. For this, a mirror of diameter 1.2 m and mass 84.7 kg having 18 support points is selected, and various cases of support force variations are simulated. Sensitivity analysis results show that for ensuring repeatability of ±3nm in surface figure measurements, the pneumatic metrology mount shall have a sensitivity of ±0.8N. To demonstrate the technology, first, a smaller nine-point pneumatic metrology mount but with sensitivity of ±0.8N, as required for the 1.2 m diameter 84.7 kg mirror, and having a provision in electronics to expand to 25 support points, is realized and validated by testing with an aluminum disk and a spherical mirror. Testing with an aluminum disk of diameter 700 mm and mass 31.28 kg is done to demonstrate the system's capability to support a mass with required forces within a tolerance of ±0.8N. Results show that the system is able to provide the required support forces to the aluminum disk within ±0.6N for a minimum 30 min. With this confidence, then testing is done with a spherical mirror of diameter 600 mm and mass 50.8 kg, having mirror original surface figure rms of 37.5 nm, and whose surface figure measurements using the proven Hindle metrology mount were available to draw comparison with the pneumatic metrology mount. Here as well, the pneumatic metrology mount is able to support the mirror with required support forces within ±0.6N for a minimum 30 min. For three interferometric measurements of surface figure (rms), the repeatability using the pneumatic mount is 0.225 nm, which is close to and better than the 0.259 nm repeatability using the Hindle mount, and the maximum deviation from the Hindle to the pneumatic metrology mount is 0.602 nm. Both values are within the required limit of ±3nm. This paper focuses on the details of the sensitivity analysis done to arrive at specifications of the pneumatic metrology mount, the developed configuration, and the testing done to validate the developed system.