I'm also interested in TCTs, the best information that I have come across sofar is, a) the 80-90s work by Jose Sasian (now a Prof. of optics at the university of Arisona)
www.willbell.com/tm/batmj/BATM-1-C01.PDF, and b) David Stevick
http://bhs.broo.k12.wv.us/homepage/a...vick/weird.htm. On David's website he has copies of his own TCT raytrace program winspot and also an older (but very capable) dos program by Jose Sasian, that calculates aberations and also provides optimisation routines.
To clarify a point, from my reading, there is a subtle difference between an "off-axis newtonian" and a tilted "unobstructed newtonian". As previously stated an "off-axis newtonian" uses a section of a parabolic mirror. But in contrast, an "unobstructed newtonian" uses a full parabolic mirror, and a special "warped" secondary mirror to overcome astigmatism, where the secondary has different "radius of curvature" in the perpendicular saggitti and tangential planes. For the classical Yolo, this warping is provided by a mechanical warping harness (ref the now decessed Arthur S. Leonard). But Jose Sasian defined in his "telescope making" articles a simple method for grinding the secondary instead of warping it.