Abstract
The phase-induced amplitude apodization complex mask coronagraph (PIAACMC) provides an efficient way to control diffraction propagation effects caused by the central obstruction/segmented mirrors of the telescope. PIAACMC can be optimized in a way that takes into account both chromatic diffraction effects caused by the telescope obstructed aperture and the tip-tilt sensitivity of the coronagraph. As a result, unlike classic phase-induced amplitude apodization (PIAA), the PIAACMC mirror shapes are often slightly asymmetric even for an on-axis configuration and require more care in calculating off-axis shapes when an off-axis configuration is preferred. A method to design off-axis PIAA mirror shapes given an on-axis mirror design is presented. The algorithm is based on geometrical ray tracing and is able to calculate off-axis PIAA mirror shapes for an arbitrary geometry of the input and output beams. The method is demonstrated using the third generation PIAACMC design for WFIRST-AFTA telescope. Geometrical optics design issues related to the off-axis diffraction propagation effects are also discussed.
Highlights
The Wide-Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph is designed to have a powerful high contrast performance for direct imaging of exoplanets by their reflected light
Any coronagraph system considered for the WFIRST-AFTA project should solve the main design problem of removing the starlight diffracted from the telescope central obstruction
The phase-induced amplitude apodization complex mask coronagraph (PIAACMC) can be optimized in a way that takes into account both chromatic diffraction effects caused by the telescope obstructed aperture, and the tip-tilt sensitivity of the coronagraph,[3,4] and still attain the contrast of 10−9 between 2.0 and 4.0 λ∕D with 10% broadband visible light (550 nm)[4] that satisfies the WFIRST-AFTA requirements.[5]
Summary
The Wide-Field Infrared Survey Telescope-Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph is designed to have a powerful high contrast performance for direct imaging of exoplanets by their reflected light. The phase-induced amplitude apodization (PIAA) complex mask coronagraph (CMC)[1] based on lossless pupil apodization by the beam shaping concept[2] combined with a complex amplitude focal plane mask gives a solution to this problem Such a combination can be designed to concentrate the diffracted starlight in the pupil area that matches the telescope obstruction and block it with the Lyot stop that replicates the obstruction shape. The pupil asymmetry introduced by the telescope obstruction produces the system asymmetry that could be shared between the PIAA mirror shapes and the occulter (see the difference between Gen 2 and 3 designs in Ref. 3) Due to this asymmetry and Fresnel propagation effects, the procedure used to design the previous generation of off-axis PIAA optics is not suitable to design the off-axis system considered in this paper. The phase mask design/characterization is outside of the scope of this paper and is considered in a paper by Kern et al.[3]
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