At 10:10 27.09.99 -0700, Acer Victoria wrote:
>:Each lens takes in a different angle of view. An apparent small movement
on a
>:wide angle will become an apparently longer movement with a telephoto. Or
in
>:the alternative it, the telephoto, will form a star trail faster. It has
>:nothing to do with the point source image. Did I understand your question
>:correctly?
>
>Yes, that was my question...although I still don't get it.
There's no magic going on here: The sky rotates at a fixed angular rate
(about 15 degrees per hour) as seen from the earth, and you're framing a
portion of the sky with your lens and camera.
Assuming you're using a lens that covers 15 degrees (~200mm) it will take
an hour of exposure before a star has created a streak across your entire
film plane. Use a normal lens that covers ~45 degrees and it will take 3
hours before a star has travelled across your film. As you generally don't
want star trails on your pictures you must use a wide angle lens to have
long exposures. If you aim for star trails, choose any focal length you
want, but I would still prefer a fast wide angle and get a celestial pole
somewhere somewhere in the frame.
BTW, the above example assumes that you aim your camera at celestial
equator, otherwise a star will take longer to cross your field, and your
exposures can thus be longer without trails.
Happy shooting!
Thomas Bryhn
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