Primary Objective Grating Telescopy: Optical Properties and Feasibility of Applications

TL;DR

This paper establishes the theoretical basis for primary objective grating telescopy, introduces a new figure of merit called 'modified etendue', and evaluates its potential for space and ground-based astronomical applications.

Contribution

It defines the 'modified etendue' as a new metric for photon collection in POG telescopes and analyzes their optical limits and advantages over traditional designs.

Findings

Diffraction limit depends on grating length.

Atmospheric seeing impacts ground-based POG performance.

Strategies discussed for optimizing POG benefits.

Abstract

We develop the theoretical foundation for primary objective grating (POG) telescopy. In recent years, a wide range of telescope designs that collect the light over a large grating and focus it with a secondary receiving optic that is placed at grazing exodus have been proposed by Thomas D. Ditto, and are sometimes referred to as Dittoscopes. Applications include discovery and characterization of exoplanets, discovery of near-Earth asteroids, and spectroscopic surveys of the sky. These telescopes would have small aerial mass, and therefore provide a path forward to launch large telescopes into space. Because this series of telescope designs departs from traditional telescope designs, it has been difficult to evaluate which applications are most advantageous for this design. Here, we define a new figure of merit, the "modified etendue," that characterizes the photon collection capability of a POG. It is demonstrated that the diffraction limit for observations is determined by the length of the grating. We evaluate the effects of atmospheric seeing for ground-based applications and the disambiguation of position vs. wavelength in the focal plane using a second dispersing element. Finally, some strategies for fully reaping the benefits of POG optical characteristics are discussed.