Signal-to-noise ratio of phase sensing telescope interferometers

TL;DR

This paper evaluates the noise and systematic errors in phase sensing telescope interferometers, focusing on their application in adaptive optics for large telescopes and space coronagraphs, highlighting their accuracy limits and practical use cases.

Contribution

It provides a detailed analysis of the uncertainties in phase sensing TIs, including numerical models and practical examples for large telescopes and space applications.

Findings

Photon noise dominates measurement errors for telescopes larger than 10 m.

TI methods are feasible for 10-50 m telescopes depending on conditions.

High sampling of WFE maps is achievable in space-borne coronagraphs.

Abstract

This paper is the third part of a trilogy dealing with the principles, performance and limitations of what I named "Telescope-Interferometers" (TIs). The basic idea consists in transforming one telescope into a Wavefront Error (WFE) sensing device. This can be achieved in two different ways, namely the off axis and phase-shifting TIs. In both cases the Point-Spread Function (PSF) measured in the focal plane of the telescope carries information about the transmitted WFE, which is retrieved by fast and simple algorithms suitable to an Adaptive Optics (AO) regime. Herein are evaluated the uncertainties of both types of TIs, in terms of noise and systematic errors. Numerical models are developed in order to establish the dependence of driving parameters such as useful spectral range, angular size of the observed star, or detector noise on the total WFE measurement error. The latter is found particularly sensitive to photon noise, which rapidly governs the achieved accuracy for telescope diameters higher than 10 m. We study a few practical examples, showing that TI method is applicable to AO systems on telescope diameters ranging from 10 to 50 m, depending on seeing conditions and magnitude of the observed stars. We also discuss the case of a space-borne coronagraph where TI technique provides high sampling of the input WFE map.