The Thirty Meter Telescope primary mirror consists of 492 1.4 m diameter hexagonal segments. A CGH-assisted interferometric testbed has been developed to quickly and accurately measure surface figure error of the 82 different segment prescriptions. In this paper, technical aspects of the testbed will be described, including interferometer design, techniques to reduce sensitivity to vibration and turbulence, use of CGH phase fiducials for 6 degree-of-freedom alignment of the interferometer to the Test Plate, proximity sensors for segment-to-Test Plate alignment, synthetic extended source technique to mitigate coherent artifacts, characterization of instrument transfer function, and system calibration. A surface figure error map of a "Type 0" full-scale segment will also be presented.
Telescope design advancements are leading to the need for larger convex secondary elements, making the use of traditional refractive test geometries impractical. In response to requests for larger convex components, Harris has developed the Plano Holographic Aspheric Stitching Technique (PHAST)[1], a novel metrology approach that offers versatility as well as improved performance for large convex components. This approach was conceived initially for the in-process testing of the Large Synoptic Survey Telescope (LSST) M2[2], a 3.4-meter diameter convex asphere and has since been expanded to a versatile design that can be quickly modified to test multiple prescriptions with minimal cost and schedule impacts. The metrology system has facilitated the production of the largest convex optic that Harris has processed and tested.
The metrology approach is a sub-aperture stitching technique that uses a diffractive nulling element. This leverages the rapid production times of the lithography industry to reduce the lead time for test set assembly. For the most common convex component geometries, this test can be ready for use in as little as six months from receipt of specifications.
We will present the development and design of this test methodology. Existing PHAST systems are providing high resolution and accurate data while demonstrating the stability of the overall approach. In addition, the approach is capable of rapid reconfiguration to accommodate testing of multiple convex optics over a range of sizes and specifications.
The Large Synoptic Survey Telescope (LSST) large field of view is achieved through a three-lens camera system and a three-mirror optical system comprised of a unique 8.4-meter diameter monolithic primary/tertiary mirror (M1M3) and a 3.4-meter diameter secondary mirror (M2)1. The M2 is a 100mm thick meniscus convex asphere. The M2 Assembly includes a welded steel cell and a support system comprised of 72 axial and 6 tangential electromechanical actuators to control the mirror figure. The M2 Assembly (including optical polishing and integrated optical testing) is being fabricated by Harris Corporation in Rochester, NY. The summary status of this system and results are presented.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective,
three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly,
integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic
vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature
Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA
contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration
capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA
distance. Two wave front calibration tests are utilized to verify the low and Mid/High spatial frequencies, overcoming
the limitations of the standard null/hologram configuration in its ability to resolve mid and high spatial frequencies. This
paper will introduce the systems level architecture and optical test layout for the CoCOA.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective,
three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly,
integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic
vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature
Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA
contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration
capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA
distance. This paper will introduce the systems level architecture and optical layout of the CoCOA and its main
subsystems.
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