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Report from the 3.5-m Mirror Project (1Sep92)
(from KPNO, NOAO Newsletter No. 31, 1 September 1992)

As part of NOAO's contribution to the WIYN Telescope, the 3.5-m Mirror
Project is to deliver the primary mirror to the telescope. The mirror
blank, a borosilicate honeycomb casting from the University of Arizona
Mirror Laboratory, was received by NOAO in 1989 and has been employed
for tests that will be useful in general for large mirror development.
By September of 1990, the mirror's surface had been polished to a
sphere, and the mirror has been used since then to develop techniques
for controlling the temperature of the mirror and for actively
supporting the mirror to obtain the best figure.

The engineering group has investigated two approaches for actively
controlling the surface. The first, known as the wavefront approach,
uses an influence matrix for the 66 support system actuators which is
derived numerically from finite element analysis. The second, known as
the modal approach, decomposes the influence matrix with empirically
derived coefficients into specific deformation modes, and does the
calculation for only those modes known to contain useful signal. (This
method is equivalent to the wavefront method if one includes the
maximum number of modes.) With both methods, a mirror figure accurate
to 1/20 wave (rms) can be obtained in a small number of iterations. The
best figure obtained in a single iteration is about 1/16 wave.

Tests of the mirror figure using both a Hartmann screen and a
scatterplate interferometer have been completed and compared. The
Hartmann test provides an independent optical test method during
aspherizing that does not use a null lens.

Larry Daggert and the 3.5-m Mirror Group

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