Use of thermal sieve to allow optical testing of cryogenic optical systems.
Full aperture testing of large cryogenic optical systems has been impractical due to the difficulty of operating a large collimator at cryogenic temperatures. The Thermal Sieve solves this problem by acting as a thermal barrier between an ambient temperature collimator and the cryogenic system under test. The Thermal Sieve uses a set of thermally controlled baffles with array of holes that are lined up to pass the light from the collimator without degrading the wavefront, while attenuating the thermal background by nearly 4 orders of magnitude. This paper provides the theory behind the Thermal Sieve system, evaluates the optimization for its optical and thermal performance, and presents the design and analysis for a specific system.
Updated status and capabilities for the LOTIS 6.5 meter collimator
The Large Optical Test and Integration Site (LOTIS) at Lockheed Martin Space Systems Company (LMSSC) in Sunnyvale, California was designed and constructed in order to allow advanced optical testing for systems up to a maximum aperture of up to 6.5 meters in air or vacuum over a bandwidth of 0.4 to over 5 μm with a design field of view of 1.5 milliradians. Previously reported information for the LOTIS 6.5 meter diameter Collimator was based on data collected during initial testing of this device at the University of Arizona's Steward Observatory Mirror Laboratory. This paper will report progress and new results for the LOTIS Collimator as it is re-assembled and tested during its final integration into its facility at LMSSC. In addition, we will discuss Scene Projection Technology (SPT) capabilities that can be added to provide user test capabilities meeting or exceeding many of the original specifications of the Collimator, primarily in increased optical bandwidth and field-of-view. Finally, we will describe additional optical tools (e.g., interferometers and smaller collimators) that are integral to the LOTIS facility that can provide flexible optical testing options for a wide array of users.
Casting the first 8.4-m borosilicate honeycomb mirror for the Large Binocular Telescope
We report on the casting of the first 8.4 meter diameter borosilicate honeycomb mirror at the Steward Observatory Mirror Laboratory. This blank will become the world's largest monolithic glass telescope mirror, and is the first of two mirrors for the large Binocular Telescope Project. The honeycomb 8.4 meter mirror was cast from 21 tons of E6 borosilicate glass manufactured by Ohara. This glass is melted into a mold constructed of aluminosilicate fiber to produce a honeycomb structure with roughly 20% of solid density. The 1662 hexagonal voids that form the honeycomb structure are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 6.8 rpm during the casting process to produce the F/1.14 paraboloid on the front surface. This shaping minimizes the amount of glass which must be removed during the grinding process. The front faceplate of the mirror will be 28 mm thick after generating and the back faceplate will be 25 mm. The overall thickness of the finished honeycomb blank is 89 cm at the outer edge and 44 cm at the central hole. The first 8.4 meter mirror blank was cast in January 1997. During the casting, two tons of glass leaked from the mold inside the spinning furnace. After a three month annealing cycle the furnace was opened for inspection. As a result of the leakage about 2 square meters of the faceplate near one edge of the mirror was too thin to be polished. In April 1997, an additional two tons of glass was loaded on top of the intact honeycomb structure. In June 1997, after heating slowly back to the annealing temperature, this extra glass was flash melted onto the front of the blank to assure that the faceplate was of sufficient thickness. After a further three month annealing cycle, the furnace was re-opened to reveal a superb casting with low bubble content and little trace of the fusion boundary. The blank has been removed from the furnace using a fixture glued to the upper surface of the blank. It will soon be stripped of its mold material in preparation for polishing.