MMA Memo #268

Measuring the Primary Mirror Surface Using a Laser Coordinate Measuring Machine

John B. Lugten [1]

June 13, 1999

Keywords: antennas, surface accuracy, holography

I discuss the possibility of using a Helium-Neon laser coordinate measuring machine (CMM) to measure the surface of the primary mirror on the prototype ALMA (MMA/LSA) antenna. Because the ALMA primary mirror approximates a concave spheroid, it appears possible to achieve 5 to 10 micron measurement accuracy by placing the measuring head near the mirror's average center of curvature (the accuracy which can be achieved on a concave spheroid is much higher than for other surface shapes). Since the measuring head is placed relatively near the telescope, surface measurements can be made at different telescope elevations from a measurement tower of only modest height; this presents the best possibility for measuring the gravitational deformation of the primary mirror on a prototype antenna before production antennas are ordered and prior to achieving a working two element millimeter interferometer. In order to determine what accuracy is typically achieved by this system and what environmental conditions produce the best results, I propose that test measurements be made at the prototype antenna site.

[1] NRAO

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Last modified: 1999-dec-09