Dispersed-fringe-accumulation-based left-subtract-right method for fine co-phasing of a dispersed fringe sensor

Abstract

In this paper, a dispersed-fringe-accumulation (DFA)-based left-subtract-right (LSR) piston estimation method (DFA-LSR), in which the dispersed fringe image is accumulated in the dispersed direction, and then the LSR method is used to estimate the piston error, is proposed for dispersed fringe sensors (DFS) in the fine co-phasing stage. The DFS is usually used to detect the piston errors (optical path difference) between different segmented mirrors or synthetic aperture telescopes. The DFA-LSR makes up for the shortcomings of the main peak position (MPP) method, which suffers from the constant offset in the pixel counts. The analysis and experiment results show that the proposed method can keep relatively better performance even at the condition of poor signal-to-noise ratio, compared with the MPP method in fine co-phasing stage.

Building a large affordable optical-NIR telescope (I): an alternate way to handle segmented primary mirror

“…The DFS is usually considered as a phasing scheme which provides segment piston errors less precisely over a large capture range (Shi et al, 2003;Smith et al, 2003). However, recent developments have shown that this technique is not only applicable to large piston errors, but it can also be used to measure phase errors with an accuracy of few nanometers, which is required by any diffraction limited segmented mirror telescope (Zhao and Cao, 2011;Li et al, 2017). In the subsequent sub-sections we have explored two different methodologies to recover piston errors (δ), one for large (δ>λ/2) and another one for small (δ < λ/2) errors.…”

Piston Error Extraction from Dual-Wavelength Interference Patterns Using Phase Retrieval Technique

“…The above methods involve the so-called 2π-problem, which restricts the maximum piston error detection range of the segments to one wavelength. The detection range of the piston error can be extended by introducing additional wavelengths [12,13]. We propose a dual-wavelength piston error detection method.…”

“…Note that the detection range of dual-wavelength piston error extraction method is the least common multiple (equivalent wavelength) of wavelength-1 and wavelength-2 [12,13], which is defined as follows.…”

“…A variety of interference-based piston error detection methods is proposed [8][9][10][11]. Dispersed fringe sensor [12,13] and modified Shack-Hartmann sensors are interference-based piston error detection sensors. The modified Shack-Hartmann sensor is successfully applied in the Keck I/II observatory [14,15].…”

“…The introduction of dual-wavelength in the scheme overcomes the 2π ambiguities problem and expands the piston error detection range [12,13]. Two interference patterns at two wavelengths can be obtained by repeating the procedure of narrow-band phasing method in two narrow band filters of different and unequally spaced wavelengths.…”