From the first Gregory‐Coudé Telescope to Gregor

Abstract

AbstractThe historic development of the Göttingen Gregory‐Coudé Telescope is sketched. The use of a Gregory system for a solar telescope was driven by the severe disadvantages of an existing Cassegrain solar telescope. The possibility of placing a (water‐cooled) field‐stop at the prime‐focus diminishes heat stress as well as straylight on the mirrors behind the field‐stop. The measured aureole intensity of the first solar Gregory telescope, installed 1959 near Locarno, classifies that instrument as a ‘quasi coronograph’ and makes it particularly suitable for straylight‐sensitive observations. The coudé mounting with two folding flat mirrors (‘German type’) assures small and daily constant instrumental polarization. This was compensated in a first step with a co‐rotating tilted glass‐plate and a modified Bowen compensator fixed to the hour axis. In a later step, a halfwave plate between both folding flat mirrors simulates the polarimetric situation of zero‐declination. The Gregory‐Coudé Telescope was then particularly suitable for polarimetry; it operated 25 years near Locarno and finally 15 years on Tenerife.