Quote:
Originally Posted by Paramount
My question is this, looking at all the forums and the multitude of fantastic and varied images posted, how come does it appear that none of the imagers are using optmised or corrected Dall Kirkhams? There are refractors, newtonians, SCTs, RCs, Cassegrains, camera lenses, Maksutov variations but no ODKs or CDKs"
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Hi Gordon, Corrected DK's are a fairly recent phenomenon in physical form, although I remember some designs flying around on the interenet as much as 10 years ago amongst ray tracing enthusiasts.
A classic DK has typically twice the coma of the equivelent Newtonian . For example an F8 DK has the same coma as an F4 Newtonian. More importantly the field is curved in the opposite direction to a Newt, so a number of off the shelf coma correctors fro Newts won't work with them.
The corrected DK uses typically two to three elements above the focal plane to correct coma, and they are typically free of astigmatism at the edge of the field. An RC without a corrector lens set ( field flattener ) , has no coma but significant astigmatism at the edge of the field.
Corrected DK's are easier to make than RC's as they have a much less corrected elipsoidal primary instead of the RC's hyperboloid, and the secondary is spherical rather than hyperboloid. You might wonder why corrected DK's aren't covering the planet.
The answer is that to achieve this performance there are significantly tight tolerance on the lens centering and spacing. I believe 0.2 mm on centration in some cases. Only high end manufacturers can build tubes that will enable the kind of stability that the corrected DK demands. Don't expect any of the Taiwanese manufacturers to be making them any time soon
