Folded Newtonian Calculator

Mirror diameter =
Focal ratio =
Diagonal size =
Focal plane to tertiary distance =
Tertiary offset from edge of primary =

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An early example of a folded (or bent) Newtonian is Ed Danilovicz's 12 inch, featured in Sky and Telescope magazine, April 1976 pg 278.
Ross Robert's folded 16 inch was featured in the Oregon Star Party Walkabout of 2008 here
Dan Gray's bent 28 inch is a popular scope at the Oregon Star Party. The eyepiece is a couple of feet closer to the ground because of the folded design.
Pete's 25 inch folded scope as posted on the atm-free discussion group, from 2006. He's since added side panels and a tertiary mirror to angle the eyepiece directly outward.

You can search on 'low rider telescope', one recent name for this design, to find more examples.

Why a folded Newt? It's all about convenience: the eyepiece height above ground is shortened. This can make the difference between ladder and no ladder or between high up a ladder and stepping up onto a stool.
Why the tiertiary design? First of all, if you want to 'turn off' the tiertiary, then set the tiertary distance to zero. Here's why I favor adding the third mirror. I am distracted by the fear of a very expensive eyepiece undergoing an impact test against the ground to the point I can't concentrate on observing. When the scope is pointed near the zenith, the heavy eyepiece ends up pointed in a downward direction. I'm constantly overtightening the focuser's setscrew that holds the eyepiece in fear of the eyepiece sliding out. Also, my neck ends up kinked. It's not a joyful observing experience.

Mel Bartels