High Performance Piaa Coronagraphy With Complex Amplitude Focal Plane Masks

Abstract

We describe a coronagraph approach where the performance of a Phase-Induced Amplitude Apodization (PIAA) coronagraph is improved by using a partially transmissive phase-shifting focal plane mask and a Lyot stop. This approach combines the low inner working angle offered by phase mask coronagraphy, the full throughput and uncompromized angular resolution of the PIAA approach, and the design flexibility of Apodized Pupil Lyot Coronagraph. A PIAA complex mask coronagraph (PIAACMC) is fully described by the focal plane mask size, or, equivalently, its complex transmission which ranges from 0 (opaque) to −1 (phase shifting). For all values of the transmission, the PIAACMC theoretically offers full on-axis extinction and 100% throughput at large angular separations. With a pure phase focal plane mask (complex transmission = −1), the PIAACMC offers 50% throughput at 0.64 λ/D while providing total extinction of an on-axis point source. This performance is very close to the "fundamental performance limit" of coronagraphy derived from first principles. For very high contrast level, imaging performance with PIAACMC is in practice limited by the angular size of the on-axis target (usually a star). We show that this fundamental limitation must be taken into account when choosing the optimal value of the focal plane mask size in the PIAACMC design. We show that the PIAACMC enables visible imaging of Jupiter-like planets at ≈1.2 λ/D from the host star, and can therefore offer almost three times more targets than a PIAA coronagraph optimized for this type of observation. We find that for visible imaging of Earthlike planets, the PIAACMC gain over a PIAA is probably much smaller, as coronagraphic performance is then strongly constrained by stellar angular size. For observations at "low" contrast (below ≈ 10 8 ), the PIAACMC offers significant performance enhancement over PIAA. This is especially relevant for ground-based high contrast imaging systems in the near-IR, where PIAACMC enables high contrast high efficiency imaging within 1 λ/D. Manufacturing tolerances for the focal plane mask are quantified for a few representative PIAACMC designs.

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

“…The APLC type coronagraph therefore constitutes a promising option for future exoplanet direct imaging missions. APLC can be combined with PIAA as the apodization implementation to reach smaller IWA (e.g., PIAACMC; Guyon et al 2010a). Our APLCs were designed with one-dimensional apodization to consider the case of axisymmetric pupils (telescope circular aperture with large central obstruction) and can be used in the presence of aperture with struts by mitigating the diffraction effects in the Lyot plane with an optimized Lyot stop.…”

“…We here provide some examples of APLCs for different central obstruction sizes (10% and 20%) to produce 10 −10 broadband PSFs dark region, see Figure 9. These designs produce a large dark region inner radius (>5 λ 0 /D) but they can be combined with PIAA as the apodization implementation to reach small IWA (e.g., PIAACMC; Guyon et al 2010a), proving promising options for future exoplanet direct imaging missions.…”

“…The Phase-Induced Amplitude Apodization coronagraph and its derivatives (Guyon et al 2005(Guyon et al , 2010a) also achieve contrast better than 10 −7 over 2% bandwidth with arbitrary shaped telescopes by introducing a Lyot stop with the same geometry as the pupil telescope. In addition to these monochromatic designs, further broadband optimizations are expected to improve the device performance (Guyon et al 2014).…”

High performance coronagraphy for direct imaging of exoplanets

“…A good example of advantageous combination between the two groups is the Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC). It offers very high coronagraphic performance [28] by combining the advantages of the phase mask Lyot coronagraph (ability to use a small size focal plane mask), the APLC (higher contrast achievable in a Lyot coronagraph by apodizing the telescope entrance aperture) and PIAA (lossless apodization). Figure 2.…”

Implementing focal-plane phase masks optimized for real telescope apertures with SLM-based digital adaptive coronagraphy

“…Although some coronagraphs are capable of reaching angular separations well within 1 λ/D in presence of unaberrated wavefront [16,17], all existing large telescope entrance pupils actually present some kind of obstruction and support structure to hold the secondary mirror (or even additional tertiary mirror, and sometimes other optics to reach Nasmyth platforms). Such more complex entrance pupil geometries detrimentally affect wavefront quality as compared to a flat-hat pupil profile, and this even for perfect correction provided by the AO system.…”