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Session 60 - Next Generation Space Telescope.
Oral session, Tuesday, January 14
Piers 4/5,
Virtually all optical telescopes make use of the standard mirror paradigm, a metallic reflecting film on a glass substrate free of distorting forces. Glass is the preferred substrate material, giving the best balance of mechanical rigidity, temporal, thermal and chemical stability. For the very large, diffraction-limited and very lightweight mirror needed for the NGST, the paradigm breaks down. We describe a new solution, which takes advantage of active control to exploit light structures of stiffer but less stable materials. The glass is reduced to membrane thickness, correctly figured on small scale, while an underlying carbon fiber structure provides the large scale spatial rigidity and short term temporal stability. The two are rigidly linked by many screw type actuators. Long term thermal and mechanical deformations of the carbon fiber are decoupled, with occasional adjustments of the actuators to preserve the surface figure. We call this new technology GMARS (Glass Membrane with Active Rigid Support). Since very large monoliths are possible by this method, the choice between segmented or monolithic mirrors is not driven by optical fabrication limitations but can be made on grounds of reliability and costs for launch and deployment. As an example, a 6 m primary for diffraction limited resolution at 1 micron wavelength (0.03 arcsec) would use a membrane 2 mm thick and 2700 50 g actuators, yielding a total mirror system weighing 600 kg. It could be launched, fully assembled and tested, in an expanded rocket fairing to an orbit taking it as far as 3 AU from the sun for low zodiacal sky background.
A 0.5 m prototype GMARS spherical mirror has been constructed with commercial piezo driven screw actuators and 2 mm membrane. A surface accuracy of 93 nm rms was achieved, already good enough for diffraction limited imaging at wavelengths longer than 3 microns.
