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Posted by Robert May on October 25, 2002 19:12:16 UTC

The Dob is the most bang for the buck and the scopes are quite transportabe.
The refractors need to have much more accurate control of grinding the curves and many people don't tightly control the ROC of their mirrors, accepting errors of curvature of several inches as being just fine. With a refractor, a ROC of 87.5" is just that, not 89" or so.
BUT, don't let this bother you as it is just a matter of grinding until you get that ROC and the 12 micron grit will allow for movement of the ROC of that size very easily. So you spend 10 wets more than with the Dob's mirror to get that particular ROC? Also, you have to control the wedge of the two surfaces but proper grinding and a good blank minimizes that quite well.
In addition, you need to build two tools that will measure things with the ground blank, a spherometer (preferably with a .0001" long travel dial indicator) and a wedge gauge and use them properly. Both are not hard to build and not going to be expensive either.
After you get the scope together, you need to test and see if you have any spherical abberation and that is cured by aspheric figuring (same process as figuring a parabolic mirror but the amount of correction is usually less than going all the way to a parabolic surface).
It might be noted that a high quality refractor will be needing 4 different ROC curvatures rather than the 1 of the parabolic mirror of the Dob.
As a result of the above, yes the refractor is a bit more difficult than that of the Dob - you work longer on the lenses than doing the simple single surface and you measure them more accurately.

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