It’s been quite some time since I’ve bought a book about telescope making: I just have been more absorbed in the world of amateur radio lately. Still, on my trip to Portland, I noticed that James Daley had published a book on an unusual telescope: the Schupmann.
The Schupmann is an unusual telescope in that it is a refracting telescope with nearly perfect correction for chromatic aberration without using any exotic glasses. The main objective lens is a simple single element lens, which has a considerable amount of chromatic aberration. To cancel out this aberration, the light path is passed through a Mangin mirror: a meniscus lens where the back surface is coated with a reflecting coating which turns it into a second surface mirror. By carefully balancing the power of each of these elements, you can cancel out the chromatic aberration of the main lens. It’s a remarkable design, and rather uncommon. The only one I had seen was Jerry Logan’s superb 7″ instrument at the Riverside Telescope Maker’s conference.
Here are some links to the associated patent: