Holographic Imaging of Crowded Fields: High Angular Resolution Imaging with Excellent Quality at Very Low Cost

TL;DR

This paper introduces a speckle holography method optimized for crowded fields, achieving high-quality, high-resolution images comparable or superior to adaptive optics, with lower cost and complexity, suitable for large telescopes.

Contribution

The paper presents a novel speckle holography technique with iterative PSF improvement and multiple reference stars, enabling high Strehl ratios and sensitivity in crowded fields at low cost.

Findings

Achieves high Strehl ratios comparable to adaptive optics.

Outperforms lucky imaging by utilizing the entire PSF.

Applicable to existing instruments and compatible with AO systems.

Abstract

We present a method for speckle holography that is optimised for crowded fields. Its two key features are an iterativ improvement of the instantaneous Point Spread Functions (PSFs) extracted from each speckle frame and the (optional) simultaneous use of multiple reference stars. In this way, high signal-to-noise and accuracy can be achieved on the PSF for each short exposure, which results in sensitive, high-Strehl re- constructed images. We have tested our method with different instruments, on a range of targets, and from the N- to the I-band. In terms of PSF cosmetics, stability and Strehl ratio, holographic imaging can be equal, and even superior, to the capabilities of currently available Adaptive Optics (AO) systems, particularly at short near-infrared to optical wavelengths. It outperforms lucky imaging because it makes use of the entire PSF and reduces the need for frame selection, thus leading to higher Strehl and improved sensitivity. Image reconstruction a posteriori, the possibility to use multiple reference stars and the fact that these reference stars can be rather faint means that holographic imaging offers a simple way to image large, dense stellar fields near the diffraction limit of large telescopes, similar to, but much less technologically demanding than, the capabilities of a multi-conjugate adaptive optics system. The method can be used with a large range of already existing imaging instruments and can also be combined with AO imaging when the corrected PSF is unstable.