"...
you can make several minutes exposures with almost no guiding!
...
Carlos Santisteban"
Clarlos,
I'm quite new to astrophotography but how did you do that and what do you
mean by 'almost'? With 200mm I guess I would capture trails in less than a
minute or so depending on the declination of the object. Maybe with a
perfectly aligned mount and RA drive (I don't have one) I might 'almost' not
have to adjust DEC or RA axis for several minutes. Or is there a well kept
secret how to slow down earth?
Just my thoughts. I have done a few unguided shots of orion with a 135mm and
captured trails after less than a minute. Here's an image:
http://www.schiffler.net/stuff/orion1.jpg and one with less than 135mm
http://www.schiffler.net/stuff/orion2.jpg
(my scanner does an awful job on dark slides)
Jochen
Fast, wide angle lens will "slow" it down some, but you are going to
get trails if you expose for "several minutes", unguided. You can
make your own "barn door" guider that will allow longer exposures.
Barn Door Mounts
OK, you want to take longer exposures without star trails and
without spending a lot of money to do it? The answer is a Barn Door
mount, also known as a Scotch or Haig mount. This is little more
than two boards and a hinge. The bottom board is held stationary
while a camera is attached to the moveable top board. The axis of
the hinge is aligned with the celestial pole and a threaded rod is
rotated to force the top board to move. This movement counteracts
the rotation of the Earth and allows the camera to remain fixed
relative to the stars.
With the correct dimensions of the boards and the rod, a barn door
drive is suitable for exposures of up to ten minutes. Not bad for a
few dollars worth of materials! More elaborate versions of barn door
mounts have been made with motor drives, multiple hinges and guide
scopes. Some of these are capable of accurately tracking the stars
for up to thirty minutes. Plans for making barn door mounts are
available in many astronomy books and on the world wide web.
There are instructions for these in some books on astronomy and there
may be even plans on the web.
--
Winsor Crosby
Long Beach, California
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