CFHT f/4 coude echelle spectrograph, 316-g/mm 30-cm mosaic grating

Abstract

In 1989, a project was funded by the Canada-France-Hawaii Telescope Corporation to construct a coudé spectrograph with an f/4 camera and capable of using a 30 by 60-cm echelle grating as well as the three 30-cm gratings used now with the existing f/8 spectrograph.Construction cost is kept very low for a spectrograph of such large aperture by avoiding both large corrector lenses and a full-aperture cross disperser. The grating blaze efficiency is near optimum because the deviation angle (between the collimator and camera axes) is small, the camera mirror being located far from the grating next to the collimator. The echelle grating has 316 grooves/mm thus comparatively wide free spectral regions, which eases the problem of preventing overlapping orders. The disadvantage is that the spectral coverage of each order exceeds both the dimensions of available Reticon or CCD detectors and the angular coverage of the spectrograph camera. Thus the spectrograph is not suited to survey work where many spectral regions are recorded simultaneously. Its advantage is its efficiency, both optically and financially, in recording a selected spectral region at high resolution. The order sorting variablewedge grism is located in the diverging beam after the slit and moves the spectrum perpendicular to dispersion in order to center it on the detector. The camera, whose monochromatic focal ratio ( i.e. for a given spectral line) is f/4, consists of a parabolic concave mirror followed by a relatively small triplet corrector lens. The central element is combined with a right-angle reflecting prism which reflects the converging beam down out of the incident light to the detector package whose dimensions are not then limited by considerations of obstruction of light by the detector package.