Acceptance Test Results from first Gemini M1 Mirror

February 4, 1998
Mr. Larry Stepp, Optics Manager,

REOSC Optique has just completed acceptance testing of the first Gemini primary mirror, with excellent results. Larry Stepp witnessed the tests at REOSC from January 12-23, and was impressed by their thoroughness. The Gemini Project has benefited from REOSC's experience with the first three ESO VLT primary mirrors, but the Gemini specifications required additional tests beyond those required by ESO.

Several images of this testing at REOSC are included in the Photo Gallery.

Dimensional measurements confirmed that REOSC has maintained the geometric accuracy of the blank as generated by Corning. The rotational symmetry of the blank is excellent. The mirror is slightly thicker (+ 0.3 mm) than specified, and the bevels are slightly narrower (- 1 mm) than specified but neither of these will cause any problem.

REOSC performed a linear Hartmann test in two directions (along both the X- and Y-axes). The results were very encouraging:

REOSC performed full-aperture and sub-aperture interferometry to measure the surface figure of the mirror. Careful design of the test and careful control of test parameters minimized the measurement errors, but all error sources identified by either REOSC or Gemini staff were quantified, and their effect included in the evaluation of the mirror figure. Therefore we have high confidence that the following interferometry results are conservative:

REOSC evaluated the point spread function of the image produced by the mirror, with the following results:

Inspections of a number of areas (chosen by Gemini) with a Nomarski microscope showed the polished surface roughness varied from about 15 angstroms RMS near the center to about 18 angstroms RMS at the outer edge. All measurements were better than the specification, which is 20 angstroms RMS.

The Gemini specifications allowed REOSC to vary the support forces from their nominal values by up to +/- 100 newtons. The test results described above were accomplished with a support pattern that varied from nominal by less than +/- 70 newtons.

In summary, we are extremely pleased with REOSC for producing such an outstanding mirror, and we look forward to the completion of the second Gemini M1 this fall. With luck it may be even better.