KEYWORDS: Actuators, Mirrors, Space mirrors, Disk lasers, Glasses, Space telescopes, Temperature metrology, Composites, Reflectivity, Lightweight mirrors
Since 1996, a team at the University of Arizona has been designing
and fabricating lightweight, active space mirrors. These glass/composite mirrors use a thin flexible substrate for the
optical surface and an actuated composite structure for support. We present a design method that yields the best figure correction
for the lightest mass by assuming that the substrate's material
properties are the limiting parameters. The results are such that
the designer decides on a total mass budget and an aperture area,
and the algorithm provides the substrate thickness, number of
support points, and the mass distribution between the substrate
and actuators.
The University of Arizona has built a 2-m lightweight active
mirror prototype for the next generation of space telescopes. This
paper briefly reviews the mirror's opto-mechanical design, and it
describes the three different metrology systems that were used to
measure it during the actuation process. We also present a list
of lessons learned while working on this project. We conclude by
discussing one of the successful projects that has come out of
this technology.
Future space telescopes will require apertures that are larger than the current state of the art, yet fit within the exiting launch restrictions on size and mass. The mass can be reduced by using a thin flexible substrate for the optical surface and a rigid, lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. We have designed, built and tested a 0.5-m demonstration mirror for use in geosynchronous Earth-imaging systems. The mirror has an areal density of 5 kg/m2 and is the lightest mirror we have made using the thin substrate design. This paper discusses the design, fabrication and performance of the 0.5-m mirror.
Lightweight mirrors for space can be made using a thin flexible substrate for the optical surface and a rigid lightweight frame with actuators for support. The accuracy of the optical surface is actively maintained by adjusting the actuators using feedback from wavefront measurements. The University of Arizona is now in the final stages of fabricating two such mirrors. A 2-m NGST Mirror System Demonstrator, with an areal density of 13 kg/m2, is being built for NASA and will be tested at cryogenic temperatures. A 50 cm development mirror, with an areal density of only 5 kg/m2, is also being fabricated. This paper discusses the fabrication processes involved with both of these mirrors.
Future space and ground telescopes will have apertures that are increasingly larger in size. The primary mirrors for these telescopes will be so large that they cannot be fabricated, transported, and/or launched as a single entity. One solution is to build a large mirror out of smaller segments. The biggest challenge in fabricating segmented mirrors is matching the individual pieces so they form a single, continuous surface. This requirement means that the radii of curvature must all match. We present a technique for matching the relative radii of curvature for segmented mirrors, and we include an error analysis of this method.
The mass of the primary mirror has dominated the mass of larger aperture (> 1 m class) telescopes. Spaceborne telescopes have much to gain from a significant reduction in areal density. Areal density is often limited by the stiffness to weight ratio of the primary mirror. Two key factors drive this criteria: telescope structural characteristics (launch and deployment) and fabrication requirements. A new class of hybrid composite mirrors has been designed, prototyped, and fabricated to demonstrate the advantage of the high stiffness to weight ratio of carbon fiber composite materials and the superior optical fabrication for low expansion glasses. This hybrid mirror utilizes a unique `set and forget' fabrication technique. A thin meniscus of glass is mounted to a stiff composite support structure using composite flexure rods. The meniscus is lightweighted using waterjet pocket milling and is conventionally polished to a precise radius of curvature. This meniscus is then supported on the flexures and actuated to a precise figure. The flexures are fixed and the actuators are removed. The substrate is then ion figured to achieve the final figure. The areal density of this mirror is 10 kg/m2. Surface figure on a 0.25 m aperture prototype was demonstrated at better than (lambda) /4 (visible) prior to ion figuring. Two 0.6 m mirrors are under fabrication. The design of the mirror and results of the fabrication and testing will be discussed.
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