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RE: Field Rotation

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Posted by Kip Crawford on October 6, 2000 14:26:52 UTC

Doing astrophotography through a Dobs without a Equatorial platform is almost impossible. Almost. Guiding a Dobs for longer exposures is futile. The Dobsonian scope is a simplified Newtonian that uses a Altitude/Azimuth mount (left-right, up-down). To follow sidereal time you`ll need at least a EQ mount or a Equatorial platform. EQ platforms are cumbersome and expensive. Most folks who have large Dobs build their own. They are sold by a few people. However the Dobsonian can be used to shoot the Moon and the brighter planets.

Here are a few ballpark exposure times depending on your scopes focal ratio. F/3 to f/6 (fast focus)

Assuming you are using eyepiece projection:

32mm....6 seconds
26mm....5 seconds
17mm....3.5 seconds
12.5mm...2 seconds

Cresent moon:
26mm (or prime focus) 1/60 to 1/125th second
17mm 1/125 to 1/500th second

Quarter moon (half):
26mm...1/125 to 1/500
17mm...the same maybe 1/250 will do

full moon:

26mm 1/250th to 1/1000th
17mm the same but not 1/1000th
These times are without using a lunar filter. If you use one then you may have to slow down your exposure times. Shoot at different speeds and write down on paper what you did in order. The film will come back in the same order, then check your notes to see what works for you. If 1 shot comes out good out of 100 shots, then you have succeeded.


This is tricky and very hard to do. I have tried shooting Sarurn through my Dobs, only one was barely acceptable.

As for deep sky objects, using a hand guided Dobs is a waste of time and resources. You`ll need some kind of a guide. Good Luck!!

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