Advanced Concepts Studies

High Earth Orbit Telescope

In the early 1990’s NASA and the DOD undertook several studies for large heavy lift launch vehicles which were based on the current space shuttle design but featured larger payload dimensions. Based on these vehicle studies it appeared quite feasible to launch a 6-m aperture ground-assembled and fully tested UV/Optical/IR telescope into a high earth orbit.

Concept studies by NASA and the scientific community for a high earth orbit telescope were conducted in the early 1990’s.

The scientific objectives for such a telescope were similar to the science objectives of HST, with additional emphasis on the infrared wavelength region up to 10-15 mm with passive cooling of the optical system to _ 100 K. An aperture range of 4 to 10 m was specified for study purposes.

The telescope derived from these studies is a diffraction limited 6-m aperture Ritchey Chretien system with a segmented primary. The system configuration is very similar to the HST. Body pointing and beam steering provides 0.5 mas tracking accuracy. Thermal analyses showed that this telescope would operate at a temperature of 70-100 K. The low temperature can be achieved in a 100 000 km high earth orbit by passive radiative cooling of the optical telescope assembly using multiple radiation shields which are aided by a large sunshield covering one side of the structure.

The key technology for this system is the segmented active primary mirror and the associated wavefront control system. However, as has been shown in other developments, such as the construction and testing of the DOD Adaptive Large Optics Technology (ALOT) telescope (4-m aperture), this technology exists. Studies have also shown that thermal and structural perturbation requiring wavefront correction occur at very low frequency at the operating equilibrium due to the long duration steady state thermal environment, thus reducing the figure control cycles for the active mirror. Initial wavefront corrections and occasional adjustment, when the telescope dwells within short Earth shadow portions of the orbit, are probably the only required corrections. Co-location of the reaction wheels with the gyros will avoid primary bending modes of the optical telescope assembly having large negative gains which interact with the pointing control system.