High-Contrast Imaging With an Arbitrary Aperture: Active Compensation of Aperture Discontinuities

Abstract

We present a new method to achieve high-contrast images using segmented and/or on-axis telescopes. Our approach relies on using two sequential Deformable Mirrors to compensate for the large amplitude excursions in the telescope aperture due to secondary support structures and/or segment gaps. In this configuration the parameter landscape of Deformable Mirror Surfaces that yield high contrast Point Spread Functions is not linear, and non-linear methods are needed to find the true minimum in the optimization topology. We solve the highly non-linear Monge-Ampere equation that is the fundamental equation describing the physics of phase induced amplitude modulation. We determine the optimum configuration for our two sequential Deformable Mirror system and show that high-throughput and high contrast solutions can be achieved using realistic surface deformations that are accessible using existing technologies. We name this process Active Compensation of Aperture Discontinuities (ACAD). We show that for geometries similar to JWST, ACAD can attain at least 10 −7 in contrast and an order of magnitude higher for both the future Extremely Large Telescopes and on-axis architectures reminiscent of HST. We show that the converging non-linear mappings resulting from our Deformable Mirror shapes actually damp near-field diffraction artifacts in the vicinity of the discontinuities. Thus ACAD actually lowers the chromatic ringing due to diffraction by segment gaps and strut's while not amplifying the diffraction at the aperture edges beyond the Fresnel regime. This outer Fresnel ringing can be mitigated by properly designing the optical system. Consequently, ACAD is a true broadband solution to the problem of high-contrast imaging with segmented and/or on-axis apertures. We finally show that once the non-linear solution is found, fine tuning with linear methods used in wavefront control can be applied to further contrast by another order of magnitude. Generally speaking, the ACAD technique can be used to significantly improve a broad class of telescope designs for a variety of problems.

Lyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph

“…for by binary amplitude apodizers (Carlotti et al 2014), focal plane phase corrections (Ruane et al 2015a,b), or a pair of deformable mirrors (Pueyo & Norman 2013).…”

Central-Obscuration Removal Plates for Focal-Plane Phase-Mask Coronagraphs with a Centrally-Obscured Telescope

“…However, it will be difficult to design and manufacture remapping devices to remove both central obscuration and spider pattern simultaneously. Several techniques have been proposed for removing the spider pattern, such as the spider removal plate (SRP) or active correction of aperture discontinuities (ACAD) (Lozi et al 2009;Pueyo & Norman 2013).…”

Apodized Pupil Lyot Coronagraphs for Arbitrary Apertures. Iv. Reduced Inner Working Angle and Increased Robustness to Low-Order Aberrations

“…Pueyo & Norman (2013) developed a different approach to introduce a fully numerical optimization of the apodization function for a given contrast target. This new approach relies on the APLC linear properties between the entrance pupil function and the coronagraphic electric field to find designs with optimal gray transmission apodization that generates coronagraphic images with high-contrast regions.…”

“…This type of approach is well adapted to the presence of deformable mirrors (DMs) envisioned in the baseline of future exoplanet direct imagers (Shaklan & Green 2006;Krist et al 2011), for wavefront control in the field of view within a region bounded by the maximum spatial frequency controllable by the DM. With this approach, Pueyo & Norman (2013) found APLC solutions reducing starlight intensity down to a 10 −10 level within a given area in the field of view in monochromatic light.…”

“…This method was adopted on SCExAO with large support structures (Martinache & Guyon 2009). The Active Correction of Aperture Discontinuities (ACAD) method is another, recent and promising pupil remapping technique that is applicable to any type of coronagraph to mitigate diffraction struts with the use of two sequential deformable mirrors (Pueyo & Norman 2013). We here develop solutions for circularly symmetric obstructed pupils to overcome the diffraction effects of central obscuration in broadband high-contrast imaging.…”