Evaluation of a diamond-cut large-groove grating for near infrared spectroscopy

The rapid progress of large format infrared detector arrays enables us to take a fairly wide spectrum with a medium resolution by a single exposure, if a coarsely grooved echelle-type grating can be used. To produce such a spectrum onto a square detector array, it is necessary to fabricate a grating with much larger groove separation than those used in visible spectroscopy, which has so far not been available. For instance, if we envision a spectrograph which produces an echelle spectrogram in the 3 to 4 microns, a realistic solution of spectrograph design would be to employ a large-groove grating usable in the orders ranging from 20-th to 30-th, or even higher. Of course, the width of such grooves is beyond the limit of the usual ruling method. We therefore attempted to produce a grating with a 125 μm groove scale by the high precision cutting of an Aluminum alloy directly with a diamond bite. In this report, we present the test result of the machine-cut grating, and make evaluation of the diffraction efficiency by referring to the result of a numerical simulation software. The grating is now incorporated in an astronomical spectrograph which works mainly in 3 μm region (Imanishi et al 1996).