Chopping secondary mirror for a cooled infrared telescope.

A 10-cm chopping secondary for operation in the cryovacuum has been developed. It is momentum compensated and allows square wave modulation of up to 20 Hz and a beam separation of up to several minutes of arc in the sky. The low heat dissipation of 5 mW at a temperature of 4.2 K makes the chopper suitable for a liquid helium-cooled space telescope.