Modern technologies of fabrication and testing of large convex secondary mirrors

Abstract

Modern large telescopes such as TAO, LSST, TMT and EELT require 0.9m-4m monolithic convex secondary mirrors. The fabrication and testing of these large convex secondary mirrors of astronomical telescopes is getting challenging as the aperture of the mirror is getting bigger. The biggest challenge to fabricate these large convex aspheric mirrors is to measure the surface figure to a few nanometers, while maintaining the testing and fabrication cycle to be efficient to minimize the downtime. For the last a couple of decades there was huge advancement in the metrology and fabrication of large aspheric secondary mirrors. College of Optical Sciences in the University Arizona developed a full fabrication and metrology process with extremely high accuracy and efficiency for manufacturing the large convex secondary mirrors.In this paper modern metrology systems including Swing-Arm Optical Coordinate Measuring System (SOCMM) which is comparable to Interferometry and a Sub-aperture stitching interferometry scalable to a several meters have been presented. Also a Computer Controlled Fabrication Process which produces extremely fine surface figure and finish has been demonstrated. These most recent development has been applied to the fabrication and testing of 0.9m aspheric convex secondary mirror for the Tokyo Atacama Observatory's 6.5m telescope and the result has been presented.

An insight on optical metrology in manufacturing

“…Accurate and precise positioning of the sub-aperture metrology unit within the global coordinate frame is essential to adjust and compensate for the motion and tolerance of the metrology system. A custom-designed and -built sub-aperture interferometric stitching system [126] to test a convex aspheric mirror ∼1 m in diameter is shown in figure 23. In this configuration, the CGH was designed to compensate for the majority of the common aspheric wavefront departures, considering all the sub-apertures.…”

Optics technology for large-aperture space telescopes: from fabrication to final acceptance tests

“…This test is good for simple surfaces, but as the aspheric departure grows, a full aperture null becomes much less practical. Therefore, the subaperture stitching interferometry or swing-arm profilometry are becoming the most attractive options when testing modern convex secondaries [168]. When utilizing sub-aperture stitching interferometry, new data processing methods to ensure accurate data across the full aperture have been developed.…”