| Author(s) and Co-Author(s) with Affiliation: Remya B S(Indian Institute of Astrophysics), Pramod Panchal(Indian Institute of Astrophysics), Sanjeev Kumar Jha(Indian Institute of Astrophysics), Karthic Kumar(Indian Institute of Astrophysics), Alikhan Basheer(Indian Institute of Astrophysics), S. Sriram(Indian Institute of Astrophysics) |
| Abstract: The Thirty Meter Telescope (TMT) requires 492 primary mirror segments, each a 1.52-m diameter meniscus roundel polished to nanometer precision. Full-aperture interferometry is impractical for such large optics; therefore, accurate surface characterization relies on Sub-Aperture Station (SAS) metrology, where the polished roundel is measured in multiple overlapping 200-mm footprints and numerically stitched to reconstruct the full-aperture wavefront. This metrology capability is central in governing convergence, quality assurance, and final acceptance of TMT segments.
We report the successful installation, calibration, and commissioning of the SAS interferometric metrology system at the India TMT Optics Fabrication Facility (ITOFF), CREST Campus, Hoskote. The system measures spatial frequency Bands 2 –5 via stitched sub-apertures and Band-6 through microscopic interferometric mode, covering spatial period from ~200 mm down to 0.8 mm. To obtain Band 2-5 results, data is captured over a 5x5 grid covering a ~500mm x 500mm area on one quadrant of the roundel. The 25 phase maps are stitched together to produce a composite phase map from which Band 2-5 results are extracted. Band 6 metrology (0.8 mm to 50 mm spatial scale) data is captured at 15 pre-identified locations with the 200 mm interferometric footprint on the roundel. The data is analysed using PSD software to extract Band 6 results.
The Prototype roundel (SN003P), fully polished in ITOFF, was characterised using the complete SAS workflow. Stitched X+ quadrant results yielded Band-2-5 results, all within TMT specification. High-frequency Band-6, measurements across 15 gave an average value of ~2.0 nm RMS, demonstrating exceptional surface quality.
This work marks the first successful TMT roundel metrology completion in India, establishing a fully independent measurement pipeline for production segments. Sub-aperture metrology is now operational, validated against TIO reference data, and forms a key capability for scaling future polishing throughput toward full telescope production.
|
