Alan J. Penny

Radiative and Hybrid Cooling of Infrared Space Telescopes

The designs of cold space telescopes, cryogenic and radiatively cooled, are similar in most elements and both benefit from orbits distant from the Earth. In particular such orbits allow the anti-sunward side of radiatively-cooled spacecraft to be used to provide large cooling radiators for the individual radiation shields. Designs incorporating these features have predicted T tei near 20 K. The attainability of such temperatures is supported by limited practical experience (IRAS, COBE). Supplementary cooling systems (cryogens, mechanical coolers) can be advantageously combined with radiative cooling in hybrid designs to provide robustness against deterioration and yet lower temperatures for detectors, instruments, and even the whole telescope. The possibility of such major additional gains is illustrated by the Very Cold Telescope option under study for Edison, which should offer T tei ≤ 5 K for a little extra mechanical cooling capacity.

Edison program: designing the next-generation infrared space observatory

Since 1990 the Edison program has studied designs for large, long-lived IR space telescopes incorporating intensive use of radiative cooling supplemented by mechanical refrigeration. This approach, which is now generally accepted as the most likely route to achieving large aperture and long lifetimes, led to proposals to ESA in 1993 and 1994 for a 1.7m observatory telescope operating at < 20 K as a Medium-sized mission and a Cornerstone, respectively. Extension of these ideas and the application of newer technology now indicate that a Cornerstone budget and an Ariane 5 launcher could accommodate mid- to far-IR telescopes of up to perhaps 3m aperture and/or achieve telescope temperatures of a few K--thereby attaining the full long-wavelength performance of cryogenic missions--in robust designs able to maintain their performance levels (i.e. low optics temperatures) for many years. These designs, too, have potential applications as the individual elements of spatial interferometers, for example, for searching for extrasolar terrestrial planets.

Edison radiatively cooled infrared space observatory

We describe the current design for Edison, the first large radiatively-cooled infrared space observatory, now under consideration by the European Space Agency. Without the large cryogen tanks, more of the spacecraft can be filled with light-collecting optics and, of course, the observatory has no built-in lifetime. Our proposal is for a telescope with a 1.7 m primary to be launched by an Atlas, Ariane 5, or Proton. The baseline orbit for the observatory is a halo around L2, a location which allows additional radiating area to be placed anti-sunward. Models of the temperature behavior of the observatory indicate an equilibrium temperature via radiation alone of about 20 K. Use of near-future cryo-coolers may allow optical system temperatures as low as approximately 15 K. Consequently, Edison will be limited in sensitivity by the celestial thermal background at wavelengths shortward of about 60 micrometers and by celestial source confusion at longer wavelengths.