Finite element models (FEMs) are being used extensively in the design of the Thirty Meter Telescope (TMT). One such
use is in the design and analysis of the Primary Segment Assembly (PSA). Each PSA supports one primary mirror
segment on the mirror cell, as well as three actuators, which are used to control three degrees of freedom - tip, tilt, and
piston - of the mirror segment. The dynamic response of the PSA is important for two reasons: it affects the response
of the mirror to fluctuating wind forces, and high-Q modes limit the bandwidth of the control loops which drive the
actuators, and impact vibration transmissivity, thereby degrading image quality. We have completed a series of tests on
a prototype PSA, in which the dynamic response was tested. We report on the test methods used to measure the dynamic
response of the PSA alone and with candidate actuators installed, and we present comparisons between the measured
response and FEM predictions. There is good agreement between FEM predictions and measured response over the
frequency range within which the dynamic response is critical to control system design.
The primary mirror segment aberrations after shape corrections with warping harness have been identified as
the single largest error term in the Thirty Meter Telescope (TMT) image quality error budget. In order to better
understand the likely errors and how they will impact the telescope performance we have performed detailed
simulations. We first generated unwarped primary mirror segment surface shapes that met TMT specifications.
Then we used the predicted warping harness influence functions and a Shack-Hartmann wavefront sensor model
to determine estimates for the 492 corrected segment surfaces that make up the TMT primary mirror. Surface
and control parameters, as well as the number of subapertures were varied to explore the parameter space. The
corrected segment shapes were then passed to an optical TMT model built using the Jet Propulsion Laboratory
(JPL) developed Modeling and Analysis for Controlled Optical Systems (MACOS) ray-trace simulator. The
generated exit pupil wavefront error maps provided RMS wavefront error and image-plane characteristics like
the Normalized Point Source Sensitivity (PSSN). The results have been used to optimize the segment shape
correction and wavefront sensor designs as well as provide input to the TMT systems engineering error budgets.
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.
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.
This paper describes the studies performed to establish a baseline conceptual design of the Segment Support Assembly
(SSA) for the Thirty Meter Telescope (TMT) primary mirror. The SSA uses a combination of mechanical whiffletrees
for axial support, a central diaphragm for lateral support, and a whiffletree-based remote-controlled warping harness for
surface figure corrections. Axial support whiffletrees are numerically optimized to minimize the resulting gravityinduced
deformation. Although a classical central diaphragm solution was eventually adopted, several lateral support
concepts are considered. Warping harness systems are analyzed and optimized for their effectiveness at correcting
second and third order optical aberrations. Thermal deformations of the optical surface are systematically analyzed
using finite element analysis. Worst-case performance of the complete system as a result of gravity loading and
temperature variations is analyzed as a function of zenith angle using an integrated finite element model.
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