Re: Re[8]: [ATM] Chromatic effects visibility

["vladimir sacek" <vlad@copper.net>]

Hi Richard,

> Petzval radius is -2523.4498mm (-99.35") so not too far from
> flat. The lenses shouldn't be too expensive, but they do need to
> be 60mm or more diameter if they are to be far enough away from
> the focuser/diagonal. AR coating is also essential I would have
> thought, with 2 plane surfaces in the corrector.

That is somewhat more of a field curvature than I thought.
The two mirrors probably have Petzval's of about
-1000mm, since for the dublet I get nearly 1600mm.
But it is still rather mild, as you say. At 20mm off-axis
it would cause about 1/4 wave p-v defocus error. The difference vs. flat 
field would hardly be noticeable.

Considering the money/effort put into an RC, a little extra required for 
upgrading it into a near flat-field anastigmatic aplant via doublet 
corrector is well worth it.

Vlad 


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