Hi,
Mike Swain wrote:
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I'd like some info on what type of focusing aids would be necessary or just
plain useful for focal lengths in the 500mm, 1000mm and 1500mm range when
used for both astro and terrestial (bird) photos.
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A very good method to focus a camera connected to a telescope is by using
the Foucault test.
It works as follows:
Mount the camera without film to the telescope.
Point the telescope at a bright star.
Open the shutter (B) and open the camera back.
If you now look through the open shutter you will see the star as a fuzzy
image.
Now take a razor knife modified to fit at the film plane and move the razor
knife in front of the shutter. If the camera is exactly in focus (1), the
star image will disappear at one distinct point. If the star image
disappears from the oposite direction of the razor movement, the camera is
too close to the telescope because the light rays cross before they reach
your eyes (2). If the star image disappears from the same direction as the
razor movement, the camera is too far from the telescope (3).
Impossible to understand? I hope this "drawing" helps:
<-- your eye -----> telescope
razor
\ | /
\ | /
\ | /
\ /
\/
3 1 2 <----- focussed star light
/\
/ \
/ \
/ \
/ \
Bye,
Erwin Voogt
Utrecht, The Netherlands
http://members.nbci.com/_XMCM/photovoogt/index.html
(with still only a few astronomy pictures on his site... too little time to
scan too much)
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