A proposal for a low instrumental polarization coude telescope. II. The German Gregory-Coude Telescope at the Observatorio del Teide.

Sanchez Almeida, J.; Martinez Pillet, V.; Kneer, F.
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Astronomy and Astrophysics Supplement, v.113, p.359

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We have put into practice the technique to minimize the instrumental polarization (IP) of coude telescopes proposed by Martinez Pillet & Sanchez Almeida (1991): a λ/2-plate inserted into the optical path, with the proper orientation, cancels the IP. The compensation of the Gregory-Coude Telescope at the Observatorio del Teide turns out to fulfil theoretical expectations. Empirical tests at 630nm demonstrate that its IP decreases by a factor ~4. We show that the residual IP is not intrinsic to the method but it is due to the limited precision of the retarder presently used. In addition, observations indicate that the insertion of the λ/2-plate does not noticeably deteriorate the optical quality of the whole telescope. In short, this work proves the practical soundness of the λ/2-plate technique to reduce IP.