Purging flow protection of infrared telescopes.

Estimates of purge gas density in IR telescopes may be arrived at by methods developed through purge gas flow property measurements in model telescopes. Calculations show that ambient O atom attenuation can be achieved with both behind-the-mirror and ring injection configurations, in the case of the Cirris I telescope. Because the ambient O atoms are considerably more difficult to attenuate than lower-energy, scattered contaminants, it is expected that the successful rejection of all contaminant types will be possible realistic mass flows which in no case exceed the rate of Cirris I helium boil-off. Although it is found that the behind-the-mirror injection mode is the most mass-flow efficient, pressure limits imposed by such constraints as heat transfer considerations may make the ring injection mode superior.