Servo System for the Athermalisation of a Germanium Lens

This memorandum describes a simple electronically controlled servo system that was developed to athermalise the geranium lens of a radiometer operating in the infra-red waveband. The system athermalisation was demonstrated, with the result that the radiometer signal was held to within 95% of maximum for a + or - 20 C lens temperature change; this compares with a signal loss of over 90% with no athermalisation.