Design, manufacturing, and testing of phase-induced amplitude apodization and phase-shifting optics for segmented telescopes

Abstract

Context. The phase-induced amplitude apodization complex mask coronagraph (PIAACMC) is a coronagraph architecture for the direct detection of extrasolar planets. The PIAACMC can achieve close to the theoretical performance limit at small angular sepa-rations. The concept is a high-performance PIAA-based coronagraph that is sufficiently versatile to be designed for next-generation segmented and obscured telescope apertures. Aims. We present key elements of the design and manufacture of a PIAACMC for the segmented pupil experiment for exoplanet detection (SPEED) testbed. The primary components of a PIAACMC system are the PIAA optics and the complex phase-shifting focal plane mask (FPM). The most challenging part of the system to model is the error on the manufacturing of the two PIAA mirrors. Methods. In this paper, we describe the design and manufacturing of the FPM and moderate-sag PIAA optics using photolithography and etching. We present the design and fabrication of the PIAACMC, along with metrology, and an initial assessment of the PIAACMC optics efficiency. Results. Errors in the fabricated component profiles degrade the overall performance. We show that the depth errors involved are of a few tens of nanometers and a few hundred nanometers for the FPM and PIAA optics, respectively. The metrological and individual per¬formance analysis of the FPM and PIAA optics provides us with an in-depth understanding of these optical quality of the components, manufacturing error propagation, and the effects of these on performance. Because the deformable mirror (DM) location is critical in a PIAA system, we show that despite the pupil remapping effect of the PIAA optics, a dual-DM wavefront control and shaping system architecture optimized for short angular separations is operating adequately to compensate for manufacturing errors and for the dark zone generated in the focal plane. Conclusions. Because the errors involved are comparable to the wavefront error on the optics, wavefront control can compensate for them. Our measurements provide reliable models that, when used in simulations, allow us to refine component specification given the manufacturing errors at the raw coronagraphic performance level as well as after wavefront control and shaping.