Single Mode Complex Amplitude Refinement Coronagraph

(Por & Haffert, 2020)

(Haffert et al., 2020)

1. A&A

   The Single-mode Complex Amplitude Refinement (SCAR) coronagraph. I. Concept, theory, and design

   Por, E.H., and Haffert, S.Y.

   A&A 635, A55 (2020)

   abstract ADS PDF

   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.

2. A&A

   The Single-mode Complex Amplitude Refinement (SCAR) coronagraph. II. Lab verification, and toward the characterization of Proxima b

   Haffert, S.Y., Por, E.H., Keller, C.U., Kenworthy, M.A., Doelman, D.S., Snik, F., and Escuti, M.J.

   A&A 635, A56 (2020)

   abstract ADS PDF

   We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8-2.4λ/D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of 2-3 × 10-4, and the 180 SCAR reaches a null of 1 × 10-4. Their monochromatic contrast is maintained within a range of ±0.16λ/D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood.