Design for a 1--5-um cryogenic echelle spectrograph for the NASA IRTF

The design of an infrared cryogenic echelle spectrograph for use on the NASA Infrared Telescope Facility is described. The resolving power achieved over the range 1-5.4 microns is 1-40,000 with slit widths of 2.0-0.5 arcsec. The spectrograph is used in a single order with a 30-arcsec-long slit. No cross dispersion is provided because of the small number of orders that can be observed at once and the need to keep the instrument as small as possible. A closed-cycle cooler is used in lieu of cryogens in order to achieve greater reliability and ease of use at the telescope. The optical layout, the design philosophy, the modes of operation, and the construction details are provided.