Re: The Cooke Photo-Visual: Why Apochromatism Isn't Everything

From: Mitch Alsup (MitchAlsup_at_aol.com)
Date: 12/03/04 

Date: 3 Dec 2004 11:17:41 -0800

jsavard@excxn.aNOSPAMb.cdn.invalid (John Savard) wrote in message news:<41b08cbc.1761000@news.ecn.ab.ca>...
>
> Well, so what? Can't you correct a lens for spherical aberration just by
> bending it?

Given a kind of glass and a particular power, there is a continuous
spectrum of bendings that give the same power. One of these bendings
will have a minimum spherical aberation, another bending will have
zero coma. So, one cannot completely eliminate spherical with a single
lens with spherical surfaces, but one can get it rather small. One
can completely eliminate coma in a single lens. With an aspherical
surface, one can end up with an objective that is corrected for both
spherical and coma.

>
> The rule of thumb that an astronomical objective only needs to be
> corrected for third-order spherical aberration, and, if one is lucky,
> coma, is based on an assumption about how steep the curves are in that
> objective. If the curves become steep enough, one has to worry about
> fifth-order spherical aberration, spherochromatism, and the other
> third-order Seidel aberrations.

This is not completely correct. When one is attempting for the optical
prescription of a lens set to have better than 1/10 wave of total error
5th order spherical has to be addressed, and as the F/ratio is reduced
this problem (and that of spherochromatism) gets worse.

<snip>
>
> It is my suspicion, therefore, that a refracting telescope based on H.
> D. Taylor's design _would_ provide views of the heavens that were
> absolutely stunning, and such a telescope could be made today, even if
> some glass substitutions were required... but the popularity of
> apochromatic telescopes today in the market owes more to their shortness
> and its attendant convenience than to their optical merit, however
> appreciated that may be as well.

I suspect that attempting to scale the patented F/16 design into the F/8
range of modern APOs would not result in a color free telescope; and
therefore, would not end up being all that 'stunning'.

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