From: "John Sherman" 
To: 
Reply-To: "John Sherman" 






Hi Jim,

>with a paracorr i get sharp stars to the edge of my nag31


Agreed. But I have heard that the Paracorr sacrifices a bit of sharpness in
the center of the field to sharpen up the edge. I have also heard that
Naglers and most other wide-field eyepieces also sacrifice a bit of
contrast and sharpness in the center of the field to sharpen up the edge of
the field. For these reasons I have heard from experienced planet observers
that it is best to remove the Paracorr and the Nagler when doing high-power
work. I'd suggest a Plossl or an Erfle. Sacrificing the sharpness at the
center is fine when you have low power, because you can't see the
sacrifice, only the benefit. I have not done any rigid testing of this, I
have only heard from other people who have. I would appreciate if you let
us know of any results you get, if you should bother to test the idea.  (An
Erfle sacrifices the edge of the field to get the maximum sharpness in the
center, according to this supposed line of "wisdom".)

The reason I bring this up is because the question is whether there is any
"optical" benefit to a f/4 over a f/6. I still haven't heard of
one. The f/4 has to be corrected by auxiliary optics to provide as pleasing
of an image. And it requires a more expensive eyepiece to handle the wide
cone of light, as lesser eyepieces add more astigmatism than you already
have coma. It is harder to keep in focus when the seeing is waving at you.
etc, etc.

The f/4 does provide lots of practical benefits aside from the optical
quality, as has been pointed out.

I'm glad you're getting good images with your scope. We're not that far
apart, I hope we meet up someday. Will you be making a trip to the White
Mountains this autumn?

John

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