Optical Design Of An Infrared Spectrometer For Astronomical Use

Abstract

A cryogenically cooled (~70K) grating spectrometer has been designed for use at 1 to 5 μm at the focus of the UKIRT 3.8 m infrared telescope. An image of a slit (90 x 20 arcseconds) is formed on to the detectors (2-D arrays of up to 2 mm x 10 mm). Image quality is better than 35 itm (typical size of detector) and wavelength resolutions of between 100 to 50,000 are achievable. Aluminium Diamond turned optics allow the alignment to be insensitive to temperature. Spot diagrams are presented for various wavelengths from 1 to 5 μm using a real simulation of the diffraction produced by the gratings or the Echelle. The chromatic aberration effects, due to the ZnSe camera lens, on image quality and defocus is evaluated. Rotation of the slit in the focal plane is plotted as a function of the grating angle. Vignetting effects on the transmission of the system for various detectors in the 2D-array is studied and found below 20%. The effects of temperature changes on the optical performance is presented. A complete tolerance analysis including manufacturing errors of the diamond turning aspheric components and mounting errors has been performed. Effects of these errors on spot size, RMS wavefront aberrations, change in focus and lateral image displacement are presented.