Reflectivity and optical surface height requirements in a broadband coronagraph 1Contrast floor due to controllable spatial frequencies

Abstract

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraph's deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.

Active Correction of Aperture Discontinuities-Optimized Stroke Minimization. II. Optimization for Future Missions

“…More importantly, we identified that there exists a well behaved regime of DM setups for which the chromatic response of ACAD-OSM follows the theoretical predictions from Shaklan & Green (2006).…”

“…Indeed, we show in ACAD-OSM I that this technique using an optimized linear technique obtained better performance than more complex techniques that try to solve the non-linear equations (Pueyo & Norman 2013). Even more surprising, once the local minimum in contrast obtained with the ACAD-OSM method was found for heavily discontinuous apertures, we were able to verify theoretical relations between contrast and spectral bandwidth (ACAD-OSM I) made by Shaklan & Green (2006) in the context of "small" phase and amplitude aberrations. Finally, the formalism introduced in Appendix A can be applied to both "small" amplitude aberrations or discontinuities.…”

“…First, we need to understand precisely how the optimum in Fresnel number is impacted by parameters such as size of the DH, number of actuators, type of aperture, and spectral bandwidth. Second, we need to prove that this technique can be used with fixed mirror apodizing techniques (PIAA, PAVC, Guyon et al 2014;Fogarty et al 2017b) (Shaklan & Green 2006;Give'On et al 2006;Pueyo & Kasdin 2007). However, they were usually used to determine surface quality limits of the optics and/or applied to DM optical design in the proposed Michelson interferometer configuration.…”

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

“…In addition to exquisite wavefront sensing methods (e.g., Zernike wavefront sensors; Wallace et al 2011;N'Diaye et al 2013b) and control strategies (e.g., deformable mirrors based methods; Shaklan & Green 2006;Pueyo & Kasdin 2007;Pueyo et al 2009), these future instruments will rely on novel high-performance coronagraphs to offer broadband raw contrasts down to 10 −10 , enabling spectroscopy of these planetary companions.…”

“…This type of approach typically generates coronagraphic images with high-contrast regions in broadband light (dark zones), which were not observed with classical APLCs using prolate apodizations. 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.…”