Advanced Concepts Studies

A Roadmap for the Next Generation Space Telescope
Space Telescope Studies and Design Issues Overview

Concepts for advanced large space telescopes which will follow the Hubble Space Telescope (HST) have been investigated by NASA and the scientific community even while HST was still under development. Studies of the Next Generation Space Telescope are continuing today. The historically very long development times and the great investment in resources to achieve one order of magnitude improvement over HST in both resolution and sensitivity demand precise assessments of the potential options within the technological and fiscal environment which exists today.

Of the new design concepts which have been studied, the segmented contiguous mirror, phased-array telescopes, and telescopes with sparsely filled apertures have attracted the most attention for future space telescope applications. Current technology, as demonstrated by the Multimirror Telescope, indicates that it is possible to break up a normally circular telescope aperture and separate the parts in order to effectively increase the aperture diameter without increasing the collecting area. Since the cost of a space telescope has been estimated to vary roughly as the cube of the diameter, this is a very important consideration. Similar ideas have long been used in radio astronomy with remarkable success. Approaches such as these are now being examined in greater detail for advanced optical telescopes in space, as well as future ground-based telescopes such as the Very Large Telescope (VLT) in order to reduce the weight and size of the sub-apertures. This would then allow assembly in space to produce a complete telescope. As studies have shown, there are many approaches to increasing the aperture and thus angular resolution and sensitivity.

Although optical performance considerations play a major role in the selection of a telescope concept, other important aspects also become major trade criteria in the design of advanced space telescopes. It is obvious that the traditional contiguous filled circular aperture concept has excellent optical performance, allows testing of the complete optical train on the ground (although necessarily under degraded conditions because of the g-loading) avoids the complexities of orbital assembly and does not require rotation of the telescope to build an image.

Unfortunately, the physical dimensions associated with this approach could exceed the transportation-to-orbit capability of projected future launch vehicles. In a similar manner, advantages and disadvantages for other concepts must be weighted against scientific requirements and technological factors.

In order to obtain a better understanding of key technology issues which have to be resolved before serious design approaches can be advanced, several different telescope system concepts have been studied and are further analyzed by NASA. As pointed out by the Space Science Board's Task Group on Astronomy and Astrophysics in their study of major directions for space sciences during the 1995-2015 time period, astronomers will probably pursue two different approaches: high resolution through interferometry, which means a large baseline between sensors, and high "throughput" for imaging faint objects which means very large mirrors.

In concert with these recommendations, NASA and the scientific community has conducted studies of representative advanced telescope configurations to further assess a range of key technical problems inherent in both interferometric-type telescopes and large contiguous primary mirror telescopes.

The following discussions summarize some of the past studies and issues for advanced space telescopes, which have been conducted by NASA and the scientific community.