We have studied the feasibility and scientific potential of a 20 - 100 m aperture astronomical telescope at the lunar pole,
with its primary mirror made of spinning liquid at less than 100K. Such a telescope, equipped with imaging and
multiplexed spectroscopic instruments for a deep infrared survey, would be revolutionary in its power to study the
distant universe, including the formation of the first stars and their assembly into galaxies. The LLMT could be used to
follow up discoveries made with the 6 m James Webb Space Telescope, with more detailed images and spectroscopic
studies, as well as to detect objects 100 times fainter, such as the first, high-red shift stars in the early universe. Our
preliminary analysis based on SMART-1 AMIE images shows ridges and crater rims within 0.5° of the North Pole are
illuminated for at least some sun angles during lunar winter. Locations near these points may prove to be ideal for the
LLMT. Lunar dust deposited on the optics or in a thin atmosphere could be problematic. An in-situ site survey appears
necessary to resolve the dust questions.
The next generation of larger space optics will need lightweight and deployed mirror systems in order to control costs and fit within current and planned launch vehicle fairings. These will require active control based on wavefront sensing to establish and maintain their optical quality. Such control has been the enabling factor for the current generation of 8 m class ground-based telescopes, whose mirrors are either single monoliths with detailed shape control or have multiple rigid segments with control of relative position. They use actuator densities of typically a few per square meter. For active space systems it will be highly desirable to test the full deployed spacecraft in a vacuum test with a scene simulator, to validate before launch the optical performance of the complete system with its closed loop control systems. To enable such testing, the space mirror system must be designed from the start to work in a 1g as well as zero g environment. The orientation we envisage has the spacecraft system pointed at the zenith, illuminated by a downward beam collimated with reference to a full aperture liquid flat. We consider here two space mirror systems. The first has rigid segments supported by position actuators to control only rigid body motions. Since the segments under test must hold their shape with an axial 1g load and no passive flotation supports, they must be smaller than for ground systems. If made of lightweighted silicon carbide or beryllium for diffraction limited imaging in the optical, they would have to be ~ 30 cm in diameter. A mirror systems made from such segments will require about 40 actuators and wavefront sensor sub-apertures per square meter. The second system is a lightweight 3.5x8 m monolith for very high contrast imaging, as is envisaged for NASA's Terrestrial Planet Finder. High accuracy control of Fourier components down to ~ 0.2 m period is required, requiring a deformable mirror with about 4000 actuators. If the primary itself is the deformable element, and has a 1 cm thick glass meniscus facesheet weighing 600 kg, the gravity-induced quilting during testing would be about 1 nm rms, low enough for ground testing of the complete system at the desired 10-10 contrast level.
Conference Committee Involvement (2)
UV/Optical/IR Space Telescopes: Innovative Technologies and Concepts III
26 August 2007 | San Diego, California, United States
UV/Optical/IR Space Telescopes: Innovative Technologies and Concepts II
31 July 2005 | San Diego, California, United States