A PROPOSAL FOR A LOW INSTRUMENTAL POLARIZATION COUDE TELESCOPE .2. THE GERMAN GREGORY-COUDE TELESCOPE AT THE OBSERVATORIO DEL TEIDE

We have put into practice the technique to minimize the instrumental p olarization (IP) of coude telescopes proposed by Martinet Pillet & San chea Almeida (1991): a lambda/2-plate inserted into the optical path, with the proper orientation, cancels the IP. The compensation of the G regory-Coude Telescope at the Observatorio del Teide turns out to fulf il theoretical expectations. Empirical tests at 630 nm demonstrate tha t its IP decreases by a factor similar to 4 We show that the residual IP is not intrinsic to the method but it is due to the limited precisi on of the retarder presently used. In addition, observations indicate that the insertion of the lambda/2-plate does not noticeably deteriora te the optical quality of the whole telescope. In short, this work pro ves the practical soundness of the lambda/2-plate technique to reduce IP.