Kodaikanal Solar Observatory

A Grubb Parsons 60 cm diameter two-mirror fused quartz coelostat mounted on 11 m tower platform directs sunlight via a flat mirror into a 60 m long underground horizontal 'tunnel'. A 38 cm aperture f/90 achromat forms a 34 cm diameter solar image at the focal plane. The telescope has an option to mount a 20 cm achromat, which provides an f/90 beam to form a 17 cm image.

(Ref: http://en.wikipedia.org/wiki/Kodaikanal_Solar_Observatory)

Details of this telescope here: http://www.iiap.res.in/files/uploads/Inhouse07/Session7/Sivaraj-Mod.pdf