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Proceedings Paper

Calibrating IR optical densities for the Gemini Planet Imager extreme adaptive optics coronagraph apodizers
Author(s): Anand Sivaramakrishnan; Rémi Soummer; G. Lawrence Carr; Christophe Dorrer; Allen Bolognesi; Neil Zimmerman; Ben R. Oppenheimer; Robin Roberts; Alexandra Greenbaum
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Paper Abstract

High contrast imaging sometimes uses apodized masks in coronagraphs to suppress diffracted starlight from a bright source in order to observe its environs. Continuously graded opacity material and metallic half-tone dots are two possible apodizers fabrication techniques. In the latter approach if dot sizes are comparable to the wavelength of the light, surface plasmon effects can complicate the optical density (OD) vs. superficial dot density relation. OD can also be a complicated function of wavelength. We measured half-tone microdot screens' and continuous materials' transmissions. Our set-up replicated the f/ 64 optical configuration of the Gemini Planet Imager's Apodized Pupil Lyot Coronagraph pupil plane, where we plan to place our pupil plane masks. Our half-tone samples were fabricated with 2, 5, and 10 micron dot sizes, our continuous greyscale was High Energy Electron Beam Sensitive (HEBS) glass (Canyon Materials Inc.). We present optical density (OD) vs. wavelength curves for our half-tone and continuous greyscale samples 1.1 - 2.5 μm wavelength range. Direct measurements of the beam intensity in the far field using a Fourier Transform Infrared Spectrograph on Beamline U4IR at Brookhaven National Laboratory's National Synchrotron Light Source (NSLS) provided transmission spectra of test patches and apodizers. We report the on-axis IR transmission spectra through screens composed of metallic dots that are comparable in size with the wavelength of the light used, over a range of optical densities. We also measured departures from simple theory describing the array of satellite spots created by thin periodic grids in the pupil of the system. Such spots are used for photometry and astrometry in coronagraphic situations. Our results pertain to both ground and space based coronagraphs that use spatially variable attenuation, typically in focal plane or pupil plane masks.

Paper Details

Date Published: 19 August 2009
PDF: 10 pages
Proc. SPIE 7440, Techniques and Instrumentation for Detection of Exoplanets IV, 74401C (19 August 2009); doi: 10.1117/12.826955
Show Author Affiliations
Anand Sivaramakrishnan, American Museum of Natural History (United States)
Stony Brook Univ. (United States)
Rémi Soummer, Space Telescope Science Institute (United States)
G. Lawrence Carr, Brookhaven National Lab. (United States)
Christophe Dorrer, Aktiwave LLC (United States)
Allen Bolognesi, Precision Optical Imaging (United States)
Neil Zimmerman, American Museum of Natural History (United States)
Columbia Univ. (United States)
Ben R. Oppenheimer, American Museum of Natural History (United States)
Robin Roberts, American Museum of Natural History (United States)
CUNY Graduate Ctr. (United States)
Alexandra Greenbaum, Rensselaer Polytechnic Institute (United States)

Published in SPIE Proceedings Vol. 7440:
Techniques and Instrumentation for Detection of Exoplanets IV
Stuart B. Shaklan, Editor(s)

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