An Optimized Pupil Coronagraph: A New Way To Observe Extrasolar Planets This work was performed for the Jet Propulsion Laboratory, California Institute.
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Presentation on theme: "An Optimized Pupil Coronagraph: A New Way To Observe Extrasolar Planets This work was performed for the Jet Propulsion Laboratory, California Institute."— Presentation transcript:
An Optimized Pupil Coronagraph: A New Way To Observe Extrasolar Planets This work was performed for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the National Aeronautics and Space Administration. M. Littman, D. Spergel, N. J. Kasdin Princeton University The Lyot Coronagraph Focal plane += Gaussian Apodizer beforeafter Entrance PupilRe-imaged Pupil The apodizer (field occulter) blocks most of the light from the star First Focal Plane Entrance PupilRe-imaged Pupil + Lyot Stop = Camera Plane Post-stop Pupil CCD Apodizer The Lyot Stop blocks diffracted light from star more than it blocks light from neighboring objects such as nearby planets Lyot Coronagraph Performance The Lyot coronagraph is adjusted for contrast by choosing the sizes of the Gaussian spot and the Lyot stop. A contrast ratio of 10 -10 is achieved for a planet at 50 milli-arc-seconds from the star with the planet at the half power point of the Gaussian apodizer, and the Lyot stop at half the size of the entrance pupil. These conditions give a total system throughput of 3%. A Better and Simpler Way to Reduce Diffracted Light From the Star... The Gaussian Pupil Coronagraph Focal plane Diameter (D) The clear area of the pupil as a function of x is a clipped Gaussian. The “double hole” form of the pupil allows for a larger dark field. Wavelength Entrance Pupil x The Gaussian Pupil greatly reduces the effects of star-light diffraction in a large triangular field to the left and the right of the star’s central image. The Gaussian Pupil achieves 10 -10 contrast ratio in one step. An X-shaped mask can be used to prevent the bulk of the star-light from entering the instrument, thereby minimizing the effects of scattered light. Additional stages of filtering can further improve the contrast ratio. Like the Lyot coronagraph, the Gaussian pupil coronagraph is adjusted for contrast and throughput. Here adjustment is the shape and size of the pupil. A contrast ratio of 10 -10 is achieved with the planet at 4 /D (50 milli-arc-seconds for and D = 8 m) and the throughput is 30%. This figure of merit is ten times better than that of the comparable Lyot coronagraph. Gaussian Pupil Performance Comparison with theory The theoretical pattern to the right is the 2D FFT of a Gaussian pupil. The experimental pattern was obtained using a green single mode HeNe laser. The pupil was recorded on a 35mm black and clear transparency and tested using 25mm dia. optics. The Optimized Pupil Coronagraph Focal plane Diameter (D) The Gaussian pupil is optimal if the x dimension is infinite. For finite x one can look to the signal processing community for better solutions. One such solution is based on prolate spheroidal wavefunctions. Wavelength Entrance Pupil x Sun To detect our earth around our sun as viewed from a distance of ~60 lightyears requires the ability to see two objects with an intensity contrast ratio of 10 -10 and an angular separation of 50 milli-arc-seconds. Earth Here we propose a new approach to terrestrial planet detection using a pupil–based coronagraph and contrast it to a conventional Lyot coronagraph. Focal plane Diameter (D) The image in the focal plane is the spatial Fourier transform of the entrance field Wavelength Airy pattern (first null at 1.22 /D) Entrance Pupil Classical Circular Aperture On Axis Performance of Optimal Aperture Improves Null by a factor of 10 and Improves Resolution by over a factor of two. Size of Dark Region can be Adjusted by Moving to Double Pupil as Shown Below. A Tradeoff Exists between Size of Dark Region and System Throughput.