Fifty-Centimeter Monolithic Space Telescopes
Frank Ravizza | 22-FS-015
In this Feasibility Study, we examined the feasibility of constructing 50-cm aperture monolithic space telescopes. Monolithic telescopes fabricate a complete Cassegrain telescope (consisting of two mirrors) into a single substrate (the monolith). Doing so results in a telescope highly robust to launch forces and to temperature variation while in orbit because the two-mirrors are always aligned in a single block of high-purity fused silica that has a small rate of thermal expansion.
LLNL has demonstrated monoliths up to 18-cm aperture. Scaling them up to 50-cm presents fundamental technical challenges. For one, mass scales with cube of aperture diameter and will exceed 60 kg. During launch, the telescope will be subject to 60g acceleration, 36 kN of force (~8000 lbf). Additionally, the thermal management requires temperature homogeneity within its bulk glass on the order of 1K. Our technical approach for this Feasibility Study was to address three feasibility questions relating to 1) optimal optical design and fabrication of 50-cm monolithic telescopes, 2) mechanical housing necessary to constrain the telescope within the space/launch environment, and 3) the thermal management of the telescope while in orbit. At the conclusion, our team reached Technology Readiness Level 2 (TRL-2), indicating a conceptual level of readiness. Additionally, we have submitted a Record of Invention (ROI) (see IL-13754) and intend to file three patent applications through the LLNL Innnovation and Partnerships Office. Our team consulted with an optics manufacturing partner and obtained a price quotation to fabricate a 50-cm monolith. We intend to continue this research in a project titled "Proliferated Resilient Economical Monolithic Space Telescopes (PREMPT)" to elevate 50-cm monoliths to TRL-4 through laboratory validation.
50-cm aperture monoliths are expected to serve in a broad set of imaging missions from space and aircraft, from national security to fundamental discovery science. This Feasibility Study aligned with LLNL's Integrated Deterrence & Technology Competition Mission Focus Area and the Space Science and Security Directors Initiative, and it draws on the optical science and technology core competency. Specifically, 50-cm monoliths provide militarily useful ground resolution imagery from Low Earth Orbit because the monolith is sub-meter class in the visible spectrum. Twenty-first century national security challenges call for constellations of dozens or more such telescopes for tactical intelligence, surveillance, and reconnaissance (TacISR) to maintain custody of high interest vehicles on the move and to provide high-resolution space-to-space cooperative inspections. Additionally, monolithic telescopes of 50-cm aperture class can be used to detect Earth-size exoplanets and study the nature of black holes and dark matter through time domain astronomy (microlensing surveys) that require large light collection apertures and a wide field of view.