> > As in the case of the true Schmidt the correction
> > if needed should be done on the primary.
>
> This is not a good advice. The correction on primary will be good
> ONLY for paraxial rays. But in a photographic instrument, we DO
> care about rays that are inclined several degrees. For oblique rays
> "projected" zone from the corrector will miss accordingly corrected
> zone on the mirror and you'll end with twice the problem. In priciple,
> if the zone is weak, don't worry about it (unless you want really
> first class visual instrument, in which case it has to be 100% right -
> that's why I advized against SCT). If zonning is strong - sorry but
> there is NO other way but to fix the corrector.
>
> > For correctors in SCT's or Wrights which are some percentage of a
> > true Schmidt the corrector
>
> In general, correctors for Wright are at least two times stronger than
> in an equivalent Schmidt. If you fiddle with spacing (by putting the
> corrector plate closer to the primary to make it easier for secondary
> to be mounted), it gets even stronger.
>
> I would just add at the end (again) that in my opinion Schmidt and its
> derivatives are excellent photographic instruments - but for a first
> class visual use they are just too difficult to make well. Just look at
> the commercial world : there are plenty of absolutely first class
> Newtonians, Maksutovs, refractors of all sorts. But noone makes really
> goodSSCT (except _maybe_ Takahashi - and for a price !).
> If you ask a professional optician to make a telescope for you at a
> given quality level (say, better that 1/8 wave on the front), the SCT
> would be by far the most expensive of all designs !
>
> Bratislav
>
>
Reply by Bob Pfaff
I would agree that any correction on the primary ( if needed ) would
degrade the offaxis rays because the primary would not be spherical.
If the corrector is make carefully by the vacuum method no correction
will be needed to the system. There will be no zones in the corrector.
That is the advantage of vacuum deforming. If a very slight correction is
needed on the primary it would be a trade off between some spherical
aberration all over or double aberration offaxis. As I
implied this is not recommended for a gross or bad corrector.
The test that I diagramed was for a Schmidt camera ( a photographic instrument not a 1/8 wave telescope ) I agree you can not get 1/8 wave optics with a ronchi grating. The best that the 48" Schmidt at Palomar does because of seeing and film is .001" star images so we dont have to be perfect here. You do seem to agree with me or I agree you that for telescopes a flat or a star test is the way to go.
A SCT is a complex telescope and a good design. The problem with commercial SCT's is you just can not make high quality optics and sell them through a dealer for $1000.00 . Celestron just sucks the corrector over a form and grinds the top flat. Meade does uses the vacuum method but the secondaries are just picked for best fit. No correction are done on the scopes as a complete systems. To do a good job a 8" would cost $10000.00. El Camino College near where I live has a great old 16" ( about 1970 ) SCT made by Celestron. A big problem with SCT's for planetary viewing is the large secondary.
Would it not be great to have a 24" telescope only 4' long? Celestron did make some.
A question for you optical math types. In Sky and Tel Nov,1976 p384 the author talks about a Willey design. The corrector was placed directly in front of the primary and zones were measured with a Foucault test. The readings are not the same as a parabolidal mirror. No info is giving on how to calculate the zone readings. I will give credit to anybody that help me with this. Sorry for the long post Bratislav :-) Bob Pfaff