Re: [ATM] colors in the sky

["John Sherman" <atm@mail.johnspics.com>]

Hi Dave,

Thanks for writing.

>I bet you've just named the culprits -- defocus and scattering

Strangely, the old half-wave mirror had an excellent coating on it - seemingly much better than the one I have now. I don't know this for a fact, no measurements were made, but also friends of mine have commented on the difference. The images used to be brighter and have more clarity (if that's the right word), and so probably less scattering. The old coating certainly looked a lot better from a couple feet away in the daytime. But now that I have a mirror which actually focuses all the light to one spot, the colors pop out. Both mirrors were well polished out, no question there. So scattering is probably not the mechanism. 

>Notice how colors go "pastel" when a SLR camera (shucks, or a
>telescope) is defocussed 

Actually, I never noticed this in my nature photography, probably because I always try to keep it IN focus. I can understand if you have a scene full of details. Say there's a basic color with small highlights and dark spots. A bit of defocus adds the white and dark to the color - so you are adding grey to the color - that would definitely reduce saturation of the color, make it pastel, but the color is always still there.

I've been experimenting with solid colors in the day and night time. A small amount of defocus does not make a difference that I can see. In fact, looking at the sky in the day or a sign in the dark, once I've defocused I can't find focus again while looking in the eyepiece(s). I go right through it without seeing a color change. 

>and I reckon that faint extended objects rate as less colorful
>than point sources).

Sure, the colors in the Trifid are faint, so it doesn't take much to wipe them out. The moon is very bright, the colors there are distinct but still pretty close to grey, and there are more highlights and dark spots so perhaps defocus is the reason there. 

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

Sure, I'll give you the beer. When are you coming over?
We can do some 'defocussing' ....

John







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