HII/L2 Mission

3 WARM LAUNCH

Next, we propose a ``warm-launch, cooled telescope'' design concept for the new infrared satellite. This concept means that the telescope and focal plane instruments are warm when the satellite is launched, but are cooled in orbit. SIRTF also has a warm-launch, cooled telescope, but still has a vacuum vessel and a liquid helium tank to cool its focal plane instruments⁶. Here we propose a satellite with no liquid helium. The satellite does not need heavy vacuum vessels and He tanks any more, and the mass of the satellite can be reduced dramatically. Hence we can increase the size of the telescope to a level which can be never achieved with the conventional infrared telescope design.

We have two methods to cool the telescope in orbit:

1. Radiative Cooling
   As mentioned in the previous section, radiative cooling works very effectively at the S-E L2. Radiative cooling alone can cool the telescope to 20 K, which is low enough for near-infrared observations.
2. Cryogenic Mechanical Cooler
   To cool the telescope and focal plane instruments below 20 K, we propose to use cryogenic mechanical coolers. Two-stage Stirling cycle coolers are to be onboard the ASTRO-F/IRIS mission⁴. The cooler is now under extensive checkouts, and is to be flight-proven in 2003. Since the lowest temperature of the two-stage Stirling cycle is above 10 K, we are now developing a Joule-Thomson type cooler to achieve lower temperature. Figure 3 shows the test configuration and its results; we successfully demonstrated a cooling power of 15 mW at 4.8 K.

The effective radiative cooling at the S-E L2 point makes the cryogenic coolers work more efficiently. Hence, even with the cryogenic coolers, it is very important to cool most of the observations instrument by radiation.

  
Figure 3: Test results of a Joule-Thomson cooler designed for space applications. We used a two-stage Stirling cycle to cool two intermediate stages of the Joule-Thomson cycle. We put 15 mW of heat load to the coldest stage.