Re: [ATM] colors in the sky

[Woodchuck <djv@bedford.net>]

On Sat, 12 Nov 2005, John Sherman wrote:

>
> Hi List,
>
> >I said:
> >I can assure you, most deepsky objects will benefit from a
> >sharp instrument (PNs, globs, gals, nebs, etc).
>
> I am baffled by one thing, which I hope someone will explain to me. I had my 22" with a poor mirror for many years. Now I have had a sharp mirror in there for a few years. And here's what I've noticed:
>
> Having a sharp mirror greatly increases the color in objects. Why is this? Before, M42 had color, now the color is like neon. Now, Luna and the Trifid easily show their colors, where none was ever before. On nights of poor seeing, the colors disappear, the same as when I had the poor mirror. But with the sharp mirror and good seeing the color comes back.
>
> Strange, eh?
>
> John

I bet you've just named the culprits -- defocus and scattering due
either to aberrations and poor surface polish in the one case, or
poor seeing with the newer, better mirror.  (Dewing, of course,
is a gross example of "poor surface polish".)

Notice how colors go "pastel" when a SLR camera (shucks, or a
telescope) is defocussed or when a deliberate soft-focus lens is
used.  (These soft-focus lenses are no longer employed or made --
they had intentional high spherical aberration -- olde tyme portrait
photographers maintain that there is a big difference between
spherical aberration (a sharp and a soft image superimposed,
they say) versus "vaseline on the lens" or just defocus.  A certain
halo effect was created around highlights.  (In our case, a star field
is nought but highlights, so spherical will produce a halo of scattered
light, and I reckon that faint extended objects rate as less colorful
than point sources).

IOW, it's all "contrast", and nothing (betcha a beer) surpasses a "perfect"
first surface mirror (paraboloid axial image, perfectly smooth
polish and coating) for contrast.

Dave
-- 
        In each of us, there burns a soul of a woodchuck.
        In every generation a few are chosen to prove it.

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