Performance of the primary mirror center of curvature optical metrology system during cryogenic testing of the JWST Pathfinder telescope
The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point to point. Each segment has a six degree of freedom hexapod actuation system and a radius of curvature (RoC) actuation system. The full telescope will be tested at its cryogenic operating temperature at Johnson Space Center. This testing will include center of curvature measurements of the PM, using the Center of Curvature Optical Assembly (COCOA) and the Absolute Distance Meter Assembly (ADMA) The COCOA includes an interferometer, a reflective null, an interferometer null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple wavelength interferometer (MWIF) is used for alignment and phasing of the PM segments. The ADMA is used to measure, and set, the spacing between the PM and the focus of the COCOA null (i.e. the PM center of curvature) for determination of the ROC. The performance of these metrology systems was assessed during two cryogenic tests at JSC. This testing was performed using the JWST Pathfinder telescope, consisting mostly of engineering development and spare hardware. The Pathfinder PM consists of two spare segments. These tests provided the opportunity to assess how well the center of curvature optical metrology hardware, along with the software and procedures, performed using real JWST telescope hardware. This paper will describe the test setup, the testing performed, and the resulting metrology system performance. The knowledge gained and the lessons learned during this testing will be of great benefit to the accurate and efficient cryogenic testing of the JWST flight telescope.
Astronomical Telescopes Instrumentation