Optical design for IRIS2: the AAT's next infrared spectrometer

IRIS2 will provide direct imaging and low dispersion spectroscopy on the Anglo-Australian Telescope (AAT) using a 1k x 1k HgCdTe array and be capable of a future upgrade - by installing a new camera - to use a 2k x 2k array. It will be used primarily with the f/8 telescope configuration but can also be used with f/15 and E/36 configurations. The optics will be entirely transmissive with all spherical surfaces. The collimator will have four elements and the camera, five. With a 50mm collimated beam diameter and an f/2.2 camera, the Ik array will provide a field 7.6 arcmin square with pixels equivalent to 0.45 arcsec. If a 2k array is fitted later, an f/4 camera will give an 8.0 arcmin square field with pixels 0.24 arcsec square. Zemax was used to optimise the design as a multi-configuration system, so that a balance was maintained between direct imaging and spectroscopic performance and between the three atmospheric windows. For direct imaging with the f/2.2 camera, a diffraction based calculation indicates the energy inside a circle inscribed within one pixel is always better than 80% of that for a diffraction limited system. Particular care was taken to provide good imaging of the telescope pupil onto the cold stop in K.