The Single-mode Complex Amplitude Refinement (SCAR) coronagraph

Abstract

Context. The recent discovery of an Earth-mass exoplanet around the nearby star Proxima Centauri provides a prime target for the search for life on planets outside our solar system. Atmospheric characterization of these planets has been proposed by blocking the starlight with a stellar coronagraph and using a high-resolution spectrograph to search for reflected starlight off the planet. Aims. Due to the large flux ratio and small angular separation between Proxima b and its host star (≲10−7 and ≲2.2λ/D respectively; at 750 nm for an 8 m-class telescope) the coronagraph requires high starlight suppression at extremely-low inner working angles. Additionally, it must operate over a broad spectral bandwidth and under residual telescope vibrations. This allows for efficient use of spectroscopic post-processing techniques. We aim to find the global optimum of an integrated coronagraphic integral-field spectrograph. Methods. We present the Single-mode Complex Amplitude Refinement (SCAR) coronagraph that uses a microlens-fed single-mode fiber array in the focal plane downstream from a pupil-plane phase plate. The mode-filtering property of the single-mode fibers allows for the nulling of starlight on the fibers. The phase pattern in the pupil plane is specifically designed to take advantage of this mode-filtering capability. Second-order nulling on the fibers expands the spectral bandwidth and decreases the tip-tilt sensitivity of the coronagraph. Results. The SCAR coronagraph has a low inner working angle (∼1λ/D) at a contrast of < 3 × 10−5 for the six fibers surrounding the star using a sufficiently-good adaptive optics system. It can operate over broad spectral bandwidths (∼20%) and delivers high throughput (> 50% including fiber injection losses). Additionally, it is robust against tip-tilt errors (∼0.1λ/D rms). We present SCAR designs for both an unobstructed and a VLT-like pupil. Conclusions. The SCAR coronagraph is a promising candidate for exoplanet detection and characterization around nearby stars using current high-resolution imaging instruments.

Detecting life outside our solar system with a large high-contrast-imaging mission

“…Typically, extreme contrasts can only be attained in narrow spectral bandwidths (~20%), so the spectroscopic implementation depends on the AO + coronagraph implementation. In addition, single-mode fibers can be used to both feed a spectrograph, and also contribute to the overall contrast [55,118]. • Polarization techniques to mitigate the detrimental polarization-dependent aberrations of any optical system [15,127], to enable the implementation of broadband liquid-crystal-based coronagraphs [96,134,136], and to enable a measurement of exoplanet polarization to both enhance the contrast as well as provide complementary diagnostics to spectroscopy.…”

Wavefront control experiments with a single mode fiber at the High-Contrast Spectroscopy Testbed for Segmented Telescopes (HCST)

“…9 However, this is mostly true when the converged solution for the DM applies a first-order null in the image plane; a second, or higher, order null has shown to be more robust to T/T errors. 12 We are currently working on forcing EFC-SMF to automatically produce second-order nulls in all solutions. For a detailed analysis of low-order wavefront error sensitivity in EFC-SMF solutions see Coker et al (2019).…”

Baseline requirements for detecting biosignatures with the HabEx and LUVOIR mission concepts

“…In comparison, 1 λ∕D for a 4-m HabEx at 1.7 μm, where CH 4 features are, is 0.087 00 . While extremely challenging, new concepts have been proposed to address coronagraphy at sub-λ∕D, e.g., the Single-mode Complex Amplitude Refinement (SCAR) coronagraph, 25,26 and the vortex fiber nuller (VFN). Moreover, given the long exposure time that is required to search for biosignatures for a 5-pc system, observing nearby systems much closer than 10 pc will not only keep the exposure time tractable but also alleviate the inner working angle problem…”