An Immersion Grating for an Astronomical Spectrograph

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When a reflection grating is immersed in a medium instead of in air, i.e. when it is used on the rear side of a prism, a resolving power increase of n(the index of the medium) is obtained. A further increase due to the anamorphic effect is possible by inclining the entrance face of the prism. In practice an increase in resolving power of about 2.3 compared to the same grating used in air can be reached in the visible wavelength range. Larger gains are possible in the IR due to the availability of high-index materials.

The design of an anamorphic immersion échelle for the ESO MultiMode Instru-ment (EMMI) is described. With this grating it is possible to reach a resolving power of 6.104 with a 1″ slit on ESO7#x2019;s 3.5 m NTT, using a collimated beam size of just 120 mm.