Acceptance Test Results from first Gemini M1 Mirror
February 4, 1998
Mr. Larry Stepp, Optics Manager, email@example.com
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
Several images of this testing at REOSC are included in the
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
REOSC performed a linear Hartmann test in two directions (along both
the X- and Y-axes). The results were very encouraging:
- The conic constant determined by the Hartmann test and the conic
constant determined by interferometry agree to within 84 parts per
million. The measured conic constant of the mirror deviated from
nominal by 150 ppm, compared to the tolerance of 500 ppm. (The range
of adjustment of conic constant by the active support system is
several thousand ppm.) The Hartmann test is a direct ray tracing test
that does not require any null correctors, fold mirrors or other
auxiliary optics. The agreement between the Hartmann test and
interferometry guarantees the Gemini primary mirror does not suffer
from a conic constant mismatch, which has been a problem on many other
telescopes including the Hubble Space Telescope.
- The Hartmann test confirmed the radius of curvature measurement
within 3 mm. The measured radius of curvature of the mirror is 28,805
mm, 5 mm longer than nominal. This is well within the tolerance for
the radius of curvature, which is 30 mm.
- The Hartmann test also confirmed the optical surface is centered on
the geometrical dimensions of the mirror within about 0.7 mm, well
within the +/- 1 mm specified. This is unusually good, and reflects
the accurate generating work done by Corning as well as good process
control at REOSC.
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:
- The intrinsic mirror surface figure is better than 16 nm RMS.
("intrinsic" means the polishing error alone, that is, the mirror
figure on a perfect mirror support).
- The maximum peak-to-valley range is 141 nm. (If the mirror were the
diameter of the earth, the largest bump on it would be less than one
- The mirror is diffraction limited at visible (and longer)
- The bottom line is, this is the best mirror of its size ever
REOSC evaluated the point spread function of the image produced by the
mirror, with the following results:
- The mirror meets all the Gemini encircled energy specifications.
This includes specifications at 2.2 micron and 550 nm wavelengths.
The more difficult specification is at 550 nm, where we required 80%
of the light within a diameter of 0.1 arc sec. Including a correction
for high-frequency errors measured with sub-aperture interferometry,
and corrections for possible measurement errors, calculations show the
mirror will concentrate 80% of the light within 0.079 arc sec at a
wavelength of 550 nm. As far as we know, there is no other mirror of
any size that can do better than this.
- The point spread function meets the difficult Gemini specification
for satellite images at all specified wavelengths (500-3000 nm). The
highest peak in the diffraction pattern around the central image at
the most difficult wavelength is only 30% of the allowed amplitude.
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
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.