Stigmatic Plane Grating Spectrograph with Order Sorter

Abstract

A new plane-grating spectrograph has been developed to give in a compact mounting a stigmatic image, a nearly flat focal curve, and resolving powers greater than 300 000 at dispersions up to 0.2 A/mm. The performance of the instrument in the low orders is at least as good as that of a Wadsworth concave grating spectrograph of the same focal length. However, the upper wavelength limit is increased to 40 000 A or even 60 000 A, in the first order of a 7500-line/inch grating. At such settings the many overlapping orders are separated by an “order sorter,” a prism predispersor crossed with the grating dispersion. This makes it possible to obtain in a single exposure on 30 in. of plate the continuous wavelength range 2140 A to 6340 A at dispersions ranging from 0.44 A/mm to 1.1 A/mm.