Hi Wayne,
You have a couple of errors of optical physics here, which are very easily made
Quote:
Originally posted by trufflehunter
Someone please correct me if my reasoning is wrong but here's my simple theory:
Let's take two 8" mirrors both figured to the same accuracy of one-tenth of a wave. One, however, is F10. The other is half that at F5. To give a magnification of, say, 200X and another equivalent eyepiece of half the focal length (or the same eyepiece plus a 2X Barlow) in the F5 scope to give the same magnification. We have to effectively 'stretch' the focal length of the F5 scope by a factor of 2 to give the same magnification. This has the unwanted side effect of halving the benefit of the one-tenth wave error.
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This is not correct, magnification is solely a function of 2 variables and has nothing to do with the F-ratio of the scope. The 2 variables are FL of the objective and the FL of the eyepiece, the scopes aperture and F-ratio have no effect on magnification. You are not stretching the scopes focal length by using a faster scope, merely using a shorter focal length eyepiece.
Some examples:-
500FLS/10FLE = 50x,
1000FLS/20FLE = 50x,
2000FLS/40FLE=50x,
FLS=Focal Length of scope(objective)
FLE=Focal length of eyepiece
All of the above examples could relate to F5 scopes with different apertures hence they have different focal lengths yet all give 50X magnification with the appropriate eyepiece.
Quote:
Originally posted by trufflehunter
This problem is horribly complicated in an SCT. An SCT has FOUR optical surfaces of some considerable complexity in it's primary light path all of which would have to be figured to an extremely high accuracy to give good combined results. Furthermore, the primary in an SCT is about F2 or even less, so if it was figured to one-tenth of a wave it would be MUCH less than that by the time the convex secondary had its way with it and turned it into a F10, even ignoring the influence of the corrector plate. I think this is why it is so difficult to produce a really outstanding SCT.
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With the Schmidt Cassegrain having a Spherical primary it is not as difficult to produce as a parabolic mirror of the same speed, unfortunately as the primary in a SCT gets faster the field curvature gets worse, for this reason schmidt cameras have the film mounted on a curved plate.
Although there are a number of optical surfaces in a SCT they are not difficult to make per se just time consuming to make very well.
Exceptional quality SCT have been built by skilled opticians, Roland Christen from Astrophysics built a 10" as a one off which Carl Zambuto rated as 1 of the best telescopes he has ever looked through. Unfortunately the cost to make it was in excess of $US 12,000 for a 10" scope, which doesn't exactly fit the mass markets targeted by Meade and Celestron.
The downsides of a SCT is the light loss due to the large number of optical surfaces, the effective light loss due to loss of light gathering area caused by the large central obstruction and the loss of planetary contrast caused by the large central obstruction. The advantages of a SCT are the ability to fit a wide range of accessories, its compactness and ease of mounting.
In answer to Slices original question, Louie went very close to the chocolates, namely that its more difficult to make a fast optic to the same high standard as a slow optic. Noting that a 1/10th wave F5 will give the same quality images at high power as a 1/10th wave F10, your just going to need a shorter FL eyepiece which often has less eye-relief and hence less comfort to get the same power.
A couple of other points to consider:-
A fast scope is a lot more taxing on eyepieces and will require better quality eyepieces to work well.
A fast scope with a barlow may not have quite as high light transmission as a slow scope because of the elements in the barlow but this is likely to be undetectable to the eye.
A fast refractor with a barlow will have a "LOT" more chromatic aberration (false colour) than a slow refactor.
A FAST SCOPE IS A LOT MORE PORTABLE AND TRANSPORTABLE
Let us know what design and aperture of scope your thinking about Slice and I can probably be a bit more specific on what F-ratios you should be thinking about.
CS-John B