Conservation of etendue in a primary objective grating telescope

摘要

A novel telescope design that uses a primary objective grating has been shown to have valuable performance features for multiple object spectroscopy as well as a flat collector with relaxed figure tolerances suitable for membrane substrates. The commonly applied figure of merit called etendue takes on new meaning here when compared to telescopes which employ a parabolic mirror In this new telescope design, a mirror is in the secondary. One dimension of the mirror conforms to the familiar metrics for measuring field-of-view and surface area. However, in the other, an anamorphic magnification feature of plane gratings takes effect at angles of diffraction as subtended at grazing off the grating plane normal. When the secondary parabolic mirror is placed at a grazing angle, compression increases exponentially, permitting unprecedented fields-of-view, collection surface areas and aperture in one of the two dimensions. We provide an analysis that calculates the compression, resolution, and field-of-view. More importantly, we study how these parameters are conserved by a secondary parabolic mirror notwithstanding that the mirror diameter can be significantly less than the length of the primary objective grating. Zemax models are used to show that along the dimension of the grating, etendue is conserved in this telescope architecture.