[Paper Review] Apodized Pupil Lyot Coronagraph Working Without Lyot Stop

Mamadou N’Diaye, Kjetil Dohlen+1 more|arXiv (Cornell University)|Jun 3, 2008

Adaptive optics and wavefront sensing1 references3 citations

TL;DR

This paper proposes a Stop-less Lyot Coronagraph (SLLC), an apodized pupil Lyot coronagraph without a Lyot stop, for high-contrast imaging in ground-based telescopes. It achieves comparable performance to the conventional Apodized Pupil Lyot Coronagraph (APLC) under adaptive optics correction, with a mean contrast gain of Δm = 0.23 magnitudes, enabling reduced stellar peak intensity and improved dynamic range without detector saturation.

ABSTRACT

In the context of high contrast imaging, we propose to evaluate the performance of the Apodized Pupil Lyot Coronagraph (APLC) working without Lyot Stop, namely Stop-less Lyot Coronagraph (SLLC). This coronagraph is a combination of an entrance pupil apodizer and an opaque mask in the following focal plane. However, contrary to APLC, SLLC is amputated by the traditional pupil stop. Our goal is to stress the interest of using this coronagraphic solution, in particular for instruments for which the introduction of a stellar coronagraph with Lyot stop is made impossible. We estimate the intensity attenuation achieved with SLLC and carry out our study with a focus on the case of Gran Telescopio Canarias (GTC). In a first step, numerical simulations are made assuming the absence of any aberration, thereafter SLLC performance is evaluated considering AO corrected wavefronts in our approach for ground-based instruments. SLLC performance proves to be equivalent to that obtained with APLC in presence of AO compensated atmospheric turbulence images, which Strehl ratio is S=0.552 at the wavelength lambda=1.57 mu m. This coronagraph allows to remove the peak intensity of a star image and therefore, avoid detector saturation. Moreover, it helps increasing the image dynamic range. A mean contrast gain in stellar magnitudes Delta m=0.23 is obtained with SLLC whereas APLC reaches a value Delta m=0.38.

Motivation & Objective

• To investigate the feasibility of using an Apodized Pupil Lyot Coronagraph (APLC) without a Lyot stop for high-contrast imaging.
• To address instrumental constraints in ground-based telescopes where integrating a Lyot stop is impractical or impossible.
• To evaluate performance gains in contrast and dynamic range when removing the Lyot stop while retaining the apodizer and focal plane mask.
• To compare SLLC performance against conventional APLC under realistic adaptive optics (AO) conditions.
• To demonstrate that SLLC can effectively suppress stellar peak intensity and prevent detector saturation.

Proposed method

• Numerical simulations are conducted assuming ideal, aberration-free wavefronts to establish baseline performance.
• Subsequent simulations incorporate AO-corrected wavefronts to model realistic atmospheric turbulence at the Gran Telescopio Canarias (GTC).
• The SLLC design uses an entrance pupil apodizer and a focal plane opaque mask, omitting the traditional Lyot stop.
• Performance is quantified using contrast metrics and dynamic range improvements under AO correction.
• The Strehl ratio of S = 0.552 at λ = 1.57 μm is used as a benchmark for AO performance in simulations.
• Contrast and intensity suppression are evaluated by comparing SLLC with standard APLC under identical conditions.

Experimental results

Research questions

• RQ1 Can a coronagraph without a Lyot stop achieve high-contrast imaging performance comparable to the conventional APLC?
• RQ2 What is the contrast gain of the SLLC relative to APLC under AO-corrected wavefronts?
• RQ3 How does the SLLC mitigate stellar peak intensity and prevent detector saturation?
• RQ4 To what extent does the absence of a Lyot stop affect the dynamic range in high-contrast imaging?
• RQ5 Can SLLC be a viable alternative in instruments where Lyot stop integration is technically infeasible?

Key findings

• The SLLC achieves a mean contrast gain of Δm = 0.23 magnitudes, demonstrating effective suppression of stellar light.
• Performance of the SLLC is equivalent to that of the conventional APLC when wavefronts are corrected by adaptive optics.
• The Strehl ratio of S = 0.552 at λ = 1.57 μm is maintained in simulations, indicating realistic AO performance.
• The SLLC successfully reduces the peak intensity of the stellar image, preventing detector saturation.
• The coronagraph enhances image dynamic range by suppressing the central peak without requiring a Lyot stop.
• The SLLC provides a practical alternative for instruments where Lyot stop integration is not feasible, without significant performance loss.