Telescope interferometers: an alternative to classical wavefront sensors

TL;DR

This paper introduces the Telescope-Interferometer, a novel direct wavefront error sensing technique for adaptive optics, which uses PSF measurements and an added reference arm to improve real-time co-phasing of large segmented telescopes.

Contribution

It presents a new interferometric method for wavefront sensing that directly measures wavefront errors from PSF data, suitable for large telescopes and real-time applications.

Findings

Demonstrates the potential of TI for high-accuracy wavefront measurement

Analyzes the impact of photon noise on measurement accuracy

Defines the operational range of TI in adaptive optics regimes

Abstract

Several types of Wavefront Sensors (WFS) are nowadays available in the field of Adaptive Optics (AO). Generally speaking, their basic principle consists in measuring slopes or curvatures of Wavefront Errors (WFE) transmitted by a telescope, subsequently reconstructing WFEs digitally. Such process, however, does not seem to be well suited for evaluating co-phasing or piston errors of future large segmented telescopes in quasi real-time. This communication presents an original, recently proposed technique for direct WFE sensing. The principle of the device, which is named "Telescope-Interferometer" (TI), is based on the addition of a reference optical arm into the telescope pupil plane. Then incident WFEs are deduced from Point Spread Function (PSF) measurements at the telescope focal plane. Herein are described two different types of TIs, and their performance are discussed in terms of intrinsic measurement accuracy and spatial resolution. Various error sources are studied by means of numerical simulations, among which photon noise sounds the most critical. Those computations finally help to define the application range of the TI method in an AO regime, including main and auxiliary telescope diameters and magnitude of the guide star. Some practical examples of optical configurations are also described and commented.