APODIZED PUPIL LYOT CORONAGRAPHS FOR ARBITRARY APERTURES. III. QUASI-ACHROMATIC SOLUTIONS

Direct imaging and spectroscopy of young giant planets from the ground requires broadband starlight suppression with coronagraphy. It is important to minimize the coronagraph chromatic sensitivity to help remove residual speckles through post-processing of images at multiple wavelengths. The coronagraph must also be able to mitigate the effects of ground-based telescopes with central obstruction. We present new properties of the Apodized Pupil Lyot Coronagraph (APLC) that enable quasi-achromatic starlight suppression over a broad bandpass (20%) and with central obstructions. We discuss the existence of these quasi-achromatic solutions using the properties of the generalized prolate spheroidal functions, which are used to define the apodizer profile. We discuss a broadband optimization method and illustrate its parameter space in terms of inner working angle and contrast. These new APLC solutions are implemented in the Gemini Planet Imager (GPI), a new facility instrument to detect and characterize young giant planets and disks, which will be commissioned in 2011. The coronagraph design delivers a contrast better than 10–7 beyond a separation of 0.2 arcsec in the presence of Gemini's central obstruction over a 20% bandpass. The science camera is an integral field spectrograph observing in one of the Y, J, or H, or in about two-thirds of the K bandpass, at a single time. Similar solutions have also been used for the Palomar 1640 coronagraphic integral field spectrograph.