Yes 11 inch is a big refractor and something I'd like to undertake as a project using an 11 inch Plano-Convex lens I acquired a couple of years ago. The lens forms part of a
Perkin Elmer 916mm focal length f/4 Type 1 Aerial Camera lens and was used as a surveillance / spy camera in the Lockheed U2 and SR-71 aircraft and imaged on to 9 x 18 inch film. Image of the lens / camera below.
The Plano-Convex lens is just over 270mm in diameter and has a focal length, as best I can currently measure it, of 840mm and is a precision piece of glass and hardware.
Possible uses could be as...
1. An 840mm focal length ~f/3 singlet refractor, possibly used for narrowband or solar imaging.
2. An ~1020mm (Need to recheck) focal length ~f/3.8 Classical Dialyte refractor using the the 270mm diameter, 840mm focal length Plano-Convex lens with a long focal length negative / diverging lens. One I have access to is a 76mm diameter, -1250mm focal length Negative meniscus lens. The lenses would be spaced apart so that
all of the converging rays from the plano-convex lens intercept as large a diameter as possible of the 76mm diameter negative meniscus lens, and hey presto a long focal length refractor.
I have a large bore Moonlight refractor focuser which I could implement with the optics. To facilitate mounting and testing the refractor would probably need to be in some sort of large timber construction with built in optical rails to move the negative meniscus lens fore and aft and also support the 11 inch front element and focuser. Then there's collimation, etc... I could even go further and find some one to aspherize the plano convex element for improved performance as a singlet refractor.
At the moment it's a bit of a thought experiment as I have a bit too much on to devote time to it, but I'm always thinking of it and whenever I see large diameter tubes I take a good look. I need something in ~300mm. I have a short length of 200mm diameter aluminium tube (thanks MattT
) and some Parallax brand tube rings to suit, but then joining the 300 to the 200 needs to be solved.
One day.
Best
JA