After the holidays, I'll start restoring this vintage Tinsley Classic Cassegrain. Forty years ago when I was a teen, CCs were fairly popular competitors to large refractors; but with SCTs & MCTs, I wonder if today they're in the same niche as large achro refractors, given the quality and declining prices of APOs...
To expensive to make and still make as much profit on as the much easier to make SCT or MCT. Lots of complex curves if you want it to perform well both on and off axis. If you make it f/10, to get it to match the off axis performance of an SCT, you have to do a lot of work on the secondary, The obstruction could be made slightly smaller, but at a hugely more complex and difficult set of curves to grind and polish.
Here is an example. I you made a cassegrain at f/10 and used f/2 and f/5 secondary so that it would be about the same physical size as an f/10 SCT, the secondary obstuction could be made only 45mm in diameter (22%) but the level of complexity required to deal with off axis aberrations would mean that this would be a far more complicated design to grind and polish. Otherwise, the field curvature would be extremly bad. (RC = - 160 for the Cass, vs an RC+ -270mm for the SCT, and this is the standard 200mm SCT. The EdgeHD is more like RC = - 810mm, which is much flatter than you are going to get with the classical cassegrain).
Now, if you took the easier route (probably the one chosen for the scope in the pitcure), things change. By using an f/5 primary and an f/2 secondary, you have the longer tube (less compact, heavier, harder to mount than the f/10 SCT), and the curves will be far easier to grind and polish. The RC will also be a very flat -3290mm, which is about as flat as fields get, which is great for imaging.
Ah, but in this design, the central obstruction is now 76mm, or 38%. And this does not include the secondary spider vane diffraction, which now goes to total obstruction as about 40%.
Now this design could be tweaked to be somewhere in between, but unless you make the total obstruction meaninfully smaller than it would be for the SCT, then what would be the point. The only inhibitor to SCT performance at the center of the field is the obstruction size. The design othewise if 100% diffraction free, and a Cassegrain with the same size obstruction would be no better.
This is why the classical cassegrain is no longer in serial production. It is difficult to make to bring it to the same compact size as the commercial SCT, and if you abandon that compact size to make it easier to produce, you also suffer a much bigger central obstruction.
This is why you used to see so many large Cassegrains in professional observatory work. The can have a very flat field at f/10 using the f/5, f/2 forumula. You see some compact cassegrains (UT McDonald has one of these) but they have the labor involved in correcting the curves needed to tame the off axis performance issues is very high.. OK for an professional observatory, but unacceptable for mass produced instruments. Just to hard to make.
I would be curious to know the primary and secondary diameters of your scope. I would guess by looking that it is f/8 to f/10, and that the primary is maybe f/3 or f/4 and the secondary is maybe f/3 or f/4. Another words, somewhere between the two examples I used above. My bet is that the secondary is also about 30%.
Maybe I am wrong, but my bet is that this scope is about the same obstruction and performacne as a standard SCT (Flatter field though) but in a package twice as long.
And there you have it. Bigger, harder to make (which means more expensive to make), and in the end no better than a standard SCT for visual work because the obstruction would be about the same size. Would be better for imaging though, because the field would be much flatter.
I would be curious to know the primary and secondary diameters though...
Data used came from the book "Telescope Optics." If there is an error, I apologize. Just repeated data from my reference work.
Edited by Eddgie, 19 October 2014 - 09:47 AM.