Re: The "Half-Apochromat", if not the "semi-apo" has been around a long time...
From: Brian Tung (brian_at_isi.edu)
Date: 11/29/04
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Date: Mon, 29 Nov 2004 19:59:54 +0000 (UTC)
John Savard wrote:
> So I suppose we now have a standard for how good a "semi-apo" has to be;
> if the color error in a triplet apochromat is about 100 times less than
> that of an achromat, I suppose that a logarithmic scale, where a
> semi-apo has to be 10 times better than an achromat, this still wouldn't
> be too far-fetched.
Usually, the longitudinal chromatic aberration in a visual apochromat,
expressed as a fraction of the focal length, is in the neighborhood of
1 part in 20,000, give or take a factor of 2 or so, which makes it 10
times better than a typical achromat. The Meade EDs, which I think
have errors on the order of 1 part in 8,000, might make a good benchmark
for a semi-apo.
Brian Tung <brian@isi.edu>
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