Polarization-based compensation of astigmatism

Chowdhury, Dola Roy; Bhattacharya, Kaushik; Chakraborty, Ajay; Ghosh, Raja

doi:10.1364/ao.43.000750

Cited by 7 publications

(1 citation statement)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Such correction plates might be fabricated from liquid crystals polymers with spatially varying magnitude and orientation of diattenuation or retardance, similar to the vortex retarders used in coronagraphy (Clark & Breckinridge 2011;McEldowney, Shemo, & Chipman 2008;Mawet et al 2009). Wedged, spherical, and aspherical crystalline elements or element assemblies can provide a wide variety of compensating polarization aberrations (Chowdhury et al 2004). Since polarization aberrations of telescopes and fold mirrors tend to be small, spatially varying anisotropic thin films, which can only provide small retardances, could provide another path toward compensation (Hodgkinson 1998). 4.…”

Section: Polarization Aberration Mitigationmentioning

confidence: 99%

Polarization Aberrations in Astronomical Telescopes: The Point Spread Function

Breckinridge

Lam

Chipman

2015

Publications of the Astronomical Society of the Pacific

View full text Add to dashboard Cite

Detailed knowledge of the image of the point spread function (PSF) is necessary to optimize astronomical coronagraph masks and to understand potential sources of errors in astrometric measurements. The PSF for astronomical telescopes and instruments depends not only on geometric aberrations and scalar wave diffraction but also on those wavefront errors introduced by the physical optics and the polarization properties of reflecting and transmitting surfaces within the optical system. These vector wave aberrations, called polarization aberrations, result from two sources: (1) the mirror coatings necessary to make the highly reflecting mirror surfaces, and (2) the optical prescription with its inevitable non-normal incidence of rays on reflecting surfaces. The purpose of this article is to characterize the importance of polarization aberrations, to describe the analytical tools to calculate the PSF image, and to provide the background to understand how astronomical image data may be affected. To show the order of magnitude of the effects of polarization aberrations on astronomical images, a generic astronomical telescope configuration is analyzed here by modeling a fast Cassegrain telescope followed by a single 90°deviation fold mirror. All mirrors in this example use bare aluminum reflective coatings and the illumination wavelength is 800 nm. Our findings for this example telescope are: (1) The image plane irradiance distribution is the linear superposition of four PSF images: one for each of the two orthogonal polarizations and one for each of two cross-coupled polarization terms. (2) The PSF image is brighter by 9% for one polarization component compared to its orthogonal state. (3) The PSF images for two orthogonal linearly polarization components are shifted with respect to each other, causing the PSF image for unpolarized point sources to become slightly elongated (elliptical) with a centroid separation of about 0.6 mas. This is important for both astrometry and coronagraph applications. (4) Part of the aberration is a polarization-dependent astigmatism, with a magnitude of 22 milliwaves, which enlarges the PSF image. ( 5) The orthogonally polarized components of unpolarized sources contain different wavefront aberrations, which differ by approximately 32 milliwaves. This implies that a wavefront correction system cannot optimally correct the aberrations for all polarizations simultaneously. (6) The polarization aberrations couple small parts of each polarization component of the light (∼10 À4 ) into the orthogonal polarization where these components cause highly distorted secondary, or "ghost" PSF images. ( 7) The radius of the spatial extent of the 90% encircled energy of these two ghost PSF image is twice as large as the radius of the Airy diffraction pattern. Coronagraphs for terrestrial exoplanet science are expected to image objects 10 À10 , or 6 orders of magnitude less than the intensity of the instrument-induced "ghost" PSF image, which will interfere with exoplanet measurements. A polarization aberra...

show abstract