Design of the cryogenic wheel mechanisms for IRCS and NIRI

The Infrared Camera and Spectrograph (IRCS) for SUBARU and Gemini Near-infrared Imager (NIRI) instruments have a common design for all wheels, based on a modified geneva mechanism with a locking cam actuated detent pin. The geneva design, in combination with the spring loaded detent mechanism, allows the stepper motor/spur gear drive to decouple from the wheel at each aperture position. The detent mechanism positions the wheel precisely. The need for precise motor control and wheel position encoding is reduced because of the detent mechanism. Six of these mechanisms are filters wheels requiring repeatable aperture positioning. The other seven mechanisms include of a slit wheel, grism wheel, pupil mask wheel, 2 beam steerers, a focal plane mask wheel, and a beamsplitter wheel. These mechanisms require repeatable, stable and accurate positioning. The number of aperture positions for the 13 wheels range from 2 to 16. The mechanisms are aligned and tested at room temperature and operated at 60 K, requiring an athermal design, far which the modified geneva mechanism is ideally suited. This paper will discuss the prototype development and final mechanical design of specific wheel mechanisms completed for the IRCS and NIRI instruments at the Institute for Astronomy.