Has anyone built or seen a TCT?

[Omaroo]

TCT - "Tilted Component Telescope". I read about on of these a while ago and am asking if anyone has ever built one or seen one in operation? The design looks very interesting in that it doesn't have any central obstructions. There is information around that I've found that describes mirror surface radii required and their relative positioning to get it to work. At least one of the mirrors in convex. I've seen pictures of one built in the USA using 6 inch mirrors, but the author stated that he was considering trying to build a 16 or even a 20-incher in the future.

The diagram attached is not to scale, and the light paths are only tokenistic at best, but you get the idea. Lack of an obstruction means, I guess, that you gather more light on a smaller mirror.

Is this a worthy project to attempt to build? Has anyone tried? Apparently laser collimation isn't difficult as it seems - even with 4 mirrors.....

Cheers
Chris

[ving]

that certainly is an interesting design chris... are all mirrors the same size? it looks like the trade off is extra bulk. :)

[ving]

found this, but you have probably seen it already.
doesnt look as complicated as your design

http://telescopemaking.org/schief.html

[janoskiss]

AFAIK, with all these reflector designs that avoid needing a CO by deflecting the incoming light at an angle, you either use a very long focal length spherical mirror(s), or the outer portion of a parabolic mirror (which would probably be cut from a much larger complete parabolic mirror) to bring the light into focus. In either case, large apertures are not practical. You either end up with an unmanageably long focal length, or need to work with a hideously large and expensive parabolic surface.

[skies2clear]

What size aperture would you consider?
And it depends whether you would make the optics or purchase them?

You should consider the possibly the TCT design could cause distortion at the focal plane because of its off-axis nature. But I haven't verified this in the design you mentioned. I would think the TCT would give a very large focal ratio, making the telescope suitable for mainly planetary viewing. The optics would be easier to make than say an off-axis paraboloid, but if you are buying the optics, the off-axis paraboloid may be a better option.

If you want up to a moderate aperture of 8 or 9", you could consider an off-axis design, such as offered by DGM optics, who make paraboloids by cutting the outer section of a larger mirror, as Janoskiss mentioned. This should give excellent results, rivalling the best refractors...no central obstruction, no chromatic aberation, no off-axis alignment aberations, but you would still have to deal with tube currents, compared to a refractor. I'd say the off-axis paraboloid would be easier to build too, and no spider to worry about, and no need for low profile focusers either!

I think the cost of the optics from DGM is not cheap, though not prohibitive, but then a hell of a lot cheaper than Fluorite Apo's. From memory, the DGM Optics have had excellent reviews. By the way, I don't own one, just a humble newtonian.
Cheers.

[anj026]

Very Interesting Chris!

I have no personal experience but I do know of an interesting book called Amateur Telescope Making by Stephen Tonkin (Springer ISBN 1-85233-000-7). Chapter 6 descibes the construction of a 12.5 inch f20 Buchroeder Quad-Schiefspiegler for planetary CCD imaging by Terry Platt.

[ballaratdragons]

This might be easier for you to make. It's called an 'Off-Axis Unobstructed Newtonian'. :thumbsup:

[rmcpb]

All these off axis scopes use part of a MUCH larger parabolic mirror as their primary. You throw away lots of expensive glass to avoid the central obstruction. The general concensus is that while these are interesting projects as soon as you get over about 6" and into shorter focal lengths you are much better off witht he complete mirror and a relatively small central obstruction.

[okiscopey]

The big problem with this is answering people's questions about the optical design. I suppose you could have "Buchroeder Quad-Schiefspiegler" printed on a card hanging around your neck.
:lol:

[Peter Bobroff]

I have a set of masks for my 20" F/5 which makes it into 8" unobstructed.

Masks at tube entrance, directly in front of primary mirror and over the diagonal face.

Seems to work well in conditions that are not quite good enough for 20" aperture.

Have seen diffraction rings around the moons of Jupiter in this mode.

[martin kew]

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 (http://www.willbell.com/tm/batmj/BATM-1-C01.PDF), and b) David Stevick http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/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.