KEYWORDS: Planets, Diffraction, Telescopes, Light scattering, Point spread functions, Digital filtering, Stars, Exoplanets, Signal to noise ratio, Sensors
In order to explore the nature of the limits on direct extrasolar planet detection we have generated high accuracy broadband background models for several different cases. The simplest assumes an ideal diffraction limited background with shot and read noise errors. More complex models based on phase error maps drawn from real metrology data include the effects of scatter in the optical system. To these backgrounds a planet image can be added at various relative intensity levels. In the simples case, the background dominated by ideal diffraction is so smooth that a median filter is very effective at removing it locally, permitting planet detection at the limits of the flat field error of the detector. The use of more complex filters for the scatter limited case will be discussed.
It was found, on orbit, that the Hubble Space Telescope had a conic constant error in the primary mirror. The result of the error is a substantial amount of spherical aberration in the image, significantly reducing image resolution and encircled energy. Parametric phase retrieval was the method used to determine the source of error and to find the magnitude from on-board camera images. The parameters which are estimated are a set of annular Zernike polynomials which are analogous to the classical aberrations. This was one of the first practical uses of phase retrieval for on-orbit measurement. This paper contains an overview of the algorithms, how they were used and the major results.
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