Precision slit design and fabrication for the Subaru IRCS instrument

The IR Camera and Spectrograph for the Subaru telescope uses a series of reflective and transmissive slits. The width of the slits ranges from 48 micrometers to 440 micrometers . The requirements for both types of slits include sharp edge definition, good surface figure at cryogenic temperature and high reflectivity. Several different substrate materials and fabrication methods were investigated. The substrate materials considered include aluminum, copper, tungsten carbide, chromium carbide, and sapphire. The fabrication methods investigated include photo-etching micro machining using UV laser, electroforming, diamond turning, conventional polishing and electrical-discharge-machining. The pros and cons of each material and fabrication method will be described.