Passive cooling of gossamer telescopes

We discuss the rationale for gossamer telescopes and why the initial ones will be used for infrared wavelengths and this necessitates a cold telescope. The sunshield presents major problems for gossamer telescopes and may limit the potential for large IR telescopes in space. Astronomical sources of heat other than sun, moon and earth would set the telescope temperature at 4.5 K. We discuss the sunshield problem and suggest that gossamer telescopes at 1AU are more likely to be limited to approximately 10 K. We discuss the spacing of the telescope and the sunshield. The optimum spacing is 100 times the telescope size. Such spacing will require constantly firing ion engines to keep the sunshield and telescope moving round the sun with the same angular velocity. Alternately it is possible to attach the sunshield to the telescope with a compression member. This will require the telescope and sunshield to be closer together. A single layer sunshield will bring the telescope temperature to approximately 25 K. Heat sources on the telescope will limit the cooling, and so far as possible heat sources must be off-loaded to the vicinity of the sunshield.