Cold stepping drive for the ISO/LWS

Development work has begun on an interchange mechanism for the pair of Fabry-Perot interferometers proposed for the Long Wavelength Spectrometer planned as one of the focal plane instruments for the Infrared Space Observatory. The two Fabry-Perot assemblies will be mounted on a balanced wheel which is to be carried on a shaft mounted in ball bearings which have been MoS 2 treated at the European Space Tribology Laboratory. Cryogenic testing is in hand at the Institute of Cryogenics, University of Southampton, UK. A ring gear on the wheel will be driven from the output pinion of a stepping motor. At a later stage in the project a choice will be made among all available cold motors with suitable performance. In the meantime, a prototype has been built of a special cryogenic permanent magnet stepping motor intended to operate on a current of a few milliamperes. The Paper describes constructional features of the drive, together with early results for the motor prototype.