Active focal plane phase mask coronagraphy with a discrete pixelated device: I. Study of the theoretical performance trade space

Abstract

Recent advances in high-contrast exoplanet imaging instrumentation have introduced the concept of adaptive coronagraphy. For example, liquid-crystal-on-silicon spatial light modulators (SLMs) can be used as programmable phase masks or digital micro-mirror devices as configurable pupil apodizers. Adaptive coronagraphy offers the ability to adjust in real time to changing observing conditions and science goals, such as switching between blind surveys and follow-ups of known objects, or optimizing observations of multiple star systems including binaries and triples. At the same time, these active devices present challenges: finite spatial sampling, limited phase resolution, and the scalar nature of their modulation can all reduce coronagraphic performance. We look at the performance of a coronagraph utilizing pixelated discrete focal plane masks (FPMs) in function of various key parameters, notably spatial sampling, phase resolution, temporal jitter, and calibration errors. The analysis includes several common FPM designs: vortex, four-quadrant phase mask, Roddier and Roddier, dual-zone phase mask (DZPM), and azimuthal cosine phase mask (ACM). Both monochromatic and 20% broadband imaging conditions are considered, along with the absence or presence of a central obstruction from a secondary mirror in the telescope pupil, associated with variations in Lyot stop sizing. Our results provide insight into the error budget and key limiting parameters of scalar, pixelated FPM coronagraphs, for which contrast performance in ideal conditions is limited by spatial sampling and the chromaticity of the scalar phase modulation. Consequently, they are mainly relevant to ground-based high-contrast imaging, rather than to the substantially deeper contrast regimes required by future space-based observatories. Although the analysis is currently mainly motivated by SLM-based implementations, the conclusions are expected to extend to other discrete pixelated focal plane phase coronagraphs, including pattern-printed phase masks or next-generation light modulation devices.