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Re: Why Longer Focal Length for Planetary?

Subject: Re: Why Longer Focal Length for Planetary?
From: Brian Tung
Date: Mon, 18 Jul 2005 20:52:34 +0000 UTC
Newsgroups: sci.astro.amateur
Mij Adyaw wrote:
> > An F9 lens will have better color correction than an F6 one.
> 
> Is this really true for all scopes? The AP 130 F6 does not exhibit any 
> color.

You may not see any, but it is there.  It may be as small as 1 part in
20,000 (or whatever it happens to be), and you won't be able to see it
at the eyepiece in all likelihood, but it can be measured.

> Very short focal length high quality eyepieces are available and therefore 
> it is not necessary to use a barlow.

Not necessary, but it might be harder on the pocketbook. :)

-- 
Brian Tung <brian@xxxxxxx>
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