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Hi,
Three problems with imaging on a dob.
- Field rotation
- Tracking
- Prime focus.
Prime focus was already touched upon.
Field rotation will occur for longer exposures, but if your only planning on doing the moon and so forth, it will be fine. Field rotation will occur because the imaging object rotates relative to the camera sensor. Equatorial mounts (GEM's) do not have this problem as they rotate the scope with motion of the stars. A dob mount (alt/az) can not rotate the field.
Tracking (unless you have a goto dob) will be non existent so only a few seconds before you start getting star trails. Especially if you use a barlow. Limit your exposures to no more than 1-2sec with a barlow on - assuming your using a DSLR. If you've got a webcam/video camera then disregard.
The imaging train when using a barlow will be like this:
Camera -> T-ring (if DSLR) -> M42 thread adapter to 2inch (or 1.25") adapter -> Barlow -> filters (if any) -> focuser.
Another problem with the dobs is that they are designed for visual viewing. The secondary mirror is a bit small. As a result you will get uneven light distribution on the sensor, so the outer areas of an image will look darker.
For planetary, you will want a barlow. Without one your jupiter (or whatever) will be very small indeed. Alot of people use 5x barlows when doing their images, but if you have a 2x barlow, try that and see what you get.
Hope that helps.
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