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Re: [OM] Flare redux: Sun Stars?

Subject: Re: [OM] Flare redux: Sun Stars?
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Sat, 08 Dec 2001 18:22:26 +0000
At 22:03 12/8/01, Joel Wilcox added a thid image:
I've added another image with sun stars to the page I posted earlier:

http://soli.inav.net/~jdub/sunstar.html

The new image is the first one on the page. This is a sunset image in which there are two sets of sun stars created by the sun itself. I didn't take the photo with any notion that I would get sun stars. Rather I was hoping to catch the beams breaking through the clouds. I got a bit of that and a bonus with the sun stars, I think.

[snip]

Two new questions:

1. Are the beams that I was *trying* to photograph (in the sunset shot just added at the link above) also a result of diffraction of any sort?

Aperture blade diffraction?

Directly around the very edge of the sun? Yes, if you look very closely.

The more distant streaks across the sky? No. The rays spreading across most of the image are called a "glory" which is seen when the sun peeks through clouds, or is directly behind a small cloud. This occurs naturally, usually with a low angle sun near dawn or dusk with the sun ray paths travelling through more of the earths's atmosphere (relative to the observer). It has nothing to do with lens flare. It is from the filtering of the clouds that varies light intensity and light ray interaction with dust/pollutants in the earth's atmosphere. It's the intensity variation from the cloud filtering that makes it visible as streaks across the sky.

2. Since diffraction in the case of a sun star happens prior to the light's hitting the rear element of a lens, are you certain that the characteristics of the lens, or at least of the rear element, have no bearing on the photograph of the diffraction (i.e., the sun star)?

What I'm getting at in the second question is whether an inherently less sharp or flare-prone lens might affect the breadth or length of the pointing of the diffracted light in the sun star.

In fact, your image shows remarkable lack of lens flare. Lesser lenses would result in a white blob-like smear around where the sun is located. Yours has good definition of the clouds very, very close to the sun. You should be able to see a "glory" with your eyes when sun and cloud conditions are correct for it to occur. Sometimes film records it better. Our eyes record it, but our brains filter the raw data our eyes and we miss things that are really there.

If you look at the reverse side of a $1 bill you will see a glory represented behind the eye above the pyramid (reverse side of the Great Seal). The original design for the obverse side of the Great Seal has a glory emanting from behind the ring of clouds. It has since been omitted from the seal. The use of a glory (or representation of it) is also found on some ancient coats-of-arms and other symbols.

-- John


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