Optical design of infrared camera CASCAM

The optical design of the optical-infrared camera, CASCAM, developed by Purple Mountain Observatory, is described here. CASCAM employs an infrared array HAWAII-1 and a CCD, simultaneously covers the wavelength range of 0.4–2.5 μm, and is designed for imaging and polarimetric imaging with the 2.16 m and 1.26 m telescopes at Xinglong Station. The camera optics consists of a focal reducer and Offner reflection system. The results of optimization with ZEMAX software show that the design satisfies the proposed observational requirements. The camera design is implemented by the achromatic focal reducer well designed for very wide wavelength band from the optical to the near-infrared, and makes some new attempts with wedged dichroic beam splitter, unrefrigerated optical-filter wheels and a refraction dithering system.