HII/L2 Mission

1.2 Constraints of Conventional Infrared Astronomical Satellites

In spite of large demand for large infrared telescopes, it is not easy to realize this type of mission. To achieve high sensitivity, we have to cool the whole telescope and focal plane instruments down to liquid helium temperatures. Hence, all of the infrared astronomical satellites flown so far carried liquid helium. Consequently, these conventional satellites had the following severe constraints.

1. The aperture size of the telescopes is small. Large amounts of liquid helium require big tanks. Moreover, liquid helium requires heavy vacuum vessels, needed only on the ground and not in space. This makes the satellites bigger and heavier, and reduced their telescope sizes. For example, in the case of the ASTRO-F/IRIS mission, the observing instrument weighs only 35 kg but the whole cryogenic system weighs more than 400 kg.
2. The mission life is limited by the hold time of the liquid helium.
3. Observations with long integration time are very difficult, since the satellites have to avoid the radiation not only from the sun but also from the earth, otherwise their radiative heat load evaporates the liquid helium very quickly.

Thus, on the basis of the conventional design concept, it is very difficult to make an observatory type infrared astronomical mission which has a large aperture telescope and can make long-integration observations with a long mission life. We need a totally new design concept to realize the ``Infrared Astronomical Observatory'' mission.