14 September 1994 Compensation of extraordinary ray pencil astigmatism generated by a birefringent crystal in convergent beam
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Abstract
SPOT is the French Earth sensing satellite, with 3 models in orbit and the fourth model to be launched in 97. Each satellite is equipped with two multispectral cameras with a 10 meter spatial resolution. The separation of the registered spectral channels is performed by dichroic filters, with an average incidence of 30 degree(s). Since SPOT 1, in the early 80's, a thick quartz plane parallel plate is placed before the dichroic filters, to make the camera not sensitive to polarization of the observed scenes. For the new generation of SPOT 5 cameras, the resolution has been improved to 5 m, and the astigmatism introduced by the quartz crystal on the extraordinary ray pencil is no more compatible with the image quality high demanding requirements. We first determine the impact of the Quartz thick plate on the extraordinary rays for a perfectly converging beam, and then for the true beam exiting the SPOT 5 telescope. We propose to compensate the geometrical aberrations due to the birefringence by adding a negative crystal with the optic axis set parallel to that of the positive crystal. The compensating crystal is placed after the dichroic filters. We propose to use Sapphire as compensating crystal. Finally we verify the camera image quality to be in accordance with the requirements.
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Jean-Jacques P. Arnoux, Jean-Jacques P. Arnoux, } "Compensation of extraordinary ray pencil astigmatism generated by a birefringent crystal in convergent beam", Proc. SPIE 2265, Polarization Analysis and Measurement II, (14 September 1994); doi: 10.1117/12.186669; https://doi.org/10.1117/12.186669
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