This paper presents refinements to the design of the TMT primary mirror segment passive-support system that are
effective in reducing gravity print-through and thermal distortion effects. First, a novel analytical method is presented
for tuning the axial and lateral support systems in a manner that results in improved optical performance when subject to
varying gravity fields. The method utilizes counterweights attached to the whiffletrees to cancel astigmatic and comatic
errors normally resulting when the lateral support system resists transverse loads induced by gravity. Secondly, several
central diaphragm designs are presented and analyzed to assess lateral-gravity and thermal distortion performance: 1) a
simple flat diaphragm, 2) a stress-relieving diaphragm having a slotted outer rim and a circumferential convolution near
the outside diameter, and 3) a flat diaphragm having a slotted outer rim. The latter design is chosen based on results from
analytical studies which show it to have better overall optical performance in the presence of gravity and thermal
environments.
The Thirty Meter Telescope (TMT) project, a partnership between ACURA, Caltech, and the University of California, is
currently developing a 30-meter diameter optical telescope. The primary mirror will be composed of 492 low expansion
glass segments. Each segment is hexagonal, nominally measuring 1.44m across the corners. Because the TMT primary
mirror is curved (i.e. not flat) and segmented with uniform 2.5mm nominal gaps, the resulting hexagonal segment
outlines cannot all be identical. All segmentation approaches studied result in some combination of shape and size
variations. These variations range from fractions of a millimeter to several millimeters. Segmentation schemes for the
TMT primary mirror are described in some detail. Various segmentation approaches are considered, with the goal being
to minimize various measures of shape variation between segments, thereby reducing overall design complexity and
cost. Two radial scaling formulations are evaluated for their effectiveness at achieving these goals. Optimal tuning of
these formulations and detailed statistics of the resulting segment shapes are provided. Finally, we present the rationale
used for selecting the preferred segmentation approach for TMT.
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