A Space Imaging Concept Based on a 4-meter Spun-Cast Borosilicate Monolithic Primary Mirror

Lockheed Martin Corporation (LMC) tasked The University of Arizona Steward Observatory (UASO) to conduct an engineering studyto examine the feasibility of creating a 4m space telescopebased on mature borosilicate technology developed at the UASO for ground-based telescopes. UASO has completed this study and concluded that existing launchvehicles can deliver a 4m monolithic telescopesystem to a 500 km circular orbitand provide reliable imagery at NIIRS7-8. An analysis of such an imager based on a lightweight, high-performance, structured 4m primary mirrorcast from borosilicate glassis described. The relatively high CTE of this glass is used to advantage by maintaining mirror shape quality with a thermal figuring method. Placed in a 290 K thermal shroud(similar to the Hubble Space Telescope), the orbit averaged figure surface error is 6nm rms when earth-looking. Space-looking optical performance shows that a similar thermal conditioning scheme combined with a 270 K shroudachieves primary mirrordistortion of 10 nm rms surface. Analysis shows that a 3-point bipod mount will provide launchsurvivability with ample margin. The primary mirror naturally maintains its shape at 1g allowing excellent end-to-end pre- launchtesting with e.g. the LOTIS 6.5m Collimator. The telescopeincludes simple systems to measure and correct mirror shape and alignment errors incorporating technologies already proven on the LOTIS Collimator. We have sketched a notional earth-looking 4m telescopeconcept combined with a wide field TMA concept into a DELTA IV or ATLAS 552 EELV fairing. We have combined an initial analysis of launchand space performance of a special light-weighted honeycomb borosilicate mirror (areal density 95 kg/m 2 ) with public domain information on the existing launchvehicles.