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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