The designs of cold space telescopes, cryogenic and radiatively cooled
, are similar in most elements and both benefit from orbits distant fr
om the Earth. In particular such orbits allow the anti-sunward side of
radiatively-cooled spacecraft to be used to provide large cooling rad
iators for the individual radiation shields. Designs incorporating the
se features have predicted T-tel 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 pr
ovide 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 Telesco
pe option under study for Edison, which should offer T-tel less than o
r equal to 5 K for a little extra mechanical cooling capacity.