An experimental investigation of immersed gratings

Abstract

ABSTRA C T A generic problem with spectrographs equipped with conventional diffraction gratings is that the maximum attainable spectral resolution scales inversely with the telescope aperture for a fixed grating dimension and angular slit width. It has long been realized that immersed gratings, where a prism is attached to the surface of a reflection grating, offer a means to bypass this limit. We show how, for the case of the Gemini Multiobject Spectrographs, the maximum spectral resolution may be approximately doubled, or, equivalently, how the same spectral resolution may be obtained with a wider slit, resulting in improved throughput when observing extended objects. After reviewing the theory of immersed gratings, we present experimental verification of the theory and experimentally quantify two potential drawbacks: reduced throughput at blaze, and ghost images. We show that these effects are small and conclude that the benefits greatly outweigh the disadvantages.