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Design and Fabrication of an Optical System for a Balloon-borne Space Telescope Stephen Furst, Thomas Dow, Kenneth Garrard Precision Engineering Center, NC State University, Raleigh NC The NASA Balloon Experimental Twin Telescope for Infrared Interferometer (BETTII) concept1, shown in Figure 1, consists of two identical telescopes mounted onto a space frame and carried to an altitude of 130,000 ft. by a balloon. Each telescope has four optical elements – a primary mirror, a turning flat, a secondary mirror, and a tertiary mirror. The balloon-based mission brings with it many unique design challenges for designing and fabricating an optical system. During the ascent into the high atmosphere, the telescope will experience temperature changes of 250 degrees F. To minimize the effect of these changes, the entire telescope is Aluminum 6061-T6 so that the optics will all expand and contract at the same rate during this thermal cycling. Also, the telescope is tilted 13.3 degrees with respect to the horizontal, so the mirrors must be light-weighted on the backside with a honeycomb structure to minimize weight and combat gravity sag. This light-weighting complicates the design because the features on the back of the mirror can print through to the optical surfaces during machining. Figure 1: BETTII space frame truss Design and fabrication of a pair of the telescope assemblies shown in Figure 2 is the subject of this paper. The primary mirror is a segment of a 2.1 m parabolic mirror that is 651 mm off-axis with an aperture of 550 mm. This mirror which has a mass of 5 kg, is mounted to the truss and the 220 mm flat turning mirror at the right is hung from it through the thin wall “trough.” The other mirrors are complex, non-rotationally symmetric designs but are smaller (< 40 mm) and can be seen sticking through a slot at the bottom of the trough. The design challenge was to keep the overhanging load from distorting the primary while robustly supporting the turning mirror which has a mass of 2 kg and is 1m from it. This is accomplished by the combination of mounting rings that are bolted to each end of the trough and kinematically support the two mirrors. Both of the telescope assemblies are mounted to the steel/carbon fiber space frame 1 Figure 2: Telescope assembly Balloon Experimental Twin Telescope for Infrared Interferometry, S. Rinehart, Proc. SPIE, 7734, 2004 by struts attached to the mounting ring. The struts have flexures that allow the aluminum mirrors and trough to contract at a different rate than the space frame without imparting significant moments into the optic mounts. Figure 3 shows the design of the primary mirror with light-weighting ribs on the back and toroidal features on the front that form half of the kinematic mount with the V-shaped groove on the mounting ring. The pattern on the back has thicker ribs with a wide spacing to combat gravity sag and thin ribs spaces 30 mm apart to reduce the amplitude of print-through during diamond turning. Analysis shows that this design will sag by 426 nm PP when tilted 13.3 degrees, and print-through is expected to produce features less than 120 nm PP. Figure 3: Primary mirror back (left) with light-weighting ribs and front (middle) with kinematic coupling, as well as deflection due to gravity sag (right) The mirror is machined on a Nanoform 600 DTM coupled with the FLORA II fast tool servo built at the PEC. The off-axis parabolic section is machined on axis by translating the shape to the center of rotation, tilting it and finding the best fit rotationally symmetric surface with a sag of 13 mm for the DTM to follow. The 1 mm correction for the non-rotationally symmetric surface, which is the desired off-axis parabola, is simultaneously added by the FTS. The result is an optical surface with form error of < 200 nm RMS and surface finish of < 30 nm RMS. The components in the mounting structure, shown in Figure 4, are machined from single pieces of aluminum 6061-T6 and heat-treated to relieve internal stresses. The two sides of the trough are joined in the middle first, to produce a one-piece base to which the rings can be attached and then shimmed as needed to align the mirrors. The trough is simultaneously light and stiff to reduce the distortion from the overhanging flat mirror while being relatively easy to fabricate thanks to the faceted design. Figure 4: Mounting structure with thin-walled trough and coupling rings