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Mike, earlier i missed your FR question Alt/Az mounts are subject to FR, as such a motor is used to rotate the Camera to compensate for the FR.
Some amount of FR can be gotten rid of in software, if exposures are short.
The longer the exposure the more suseptable it is to
1. FR
2. Tracking errors
3. Atmospherics.
For planets 2 and 3 are important and stacking can help overcome both. As capturing more frames increases the likelihood of getting good ones.
For DSO 1, 2 and 3 are an issue. Stacking lots of shorter exposures helps avoid 1 and overcome some aspects of 2.
But for better dynamic range ( i think thats the right one) you need better compensation for 2 and 1 to achieve longer exposures. Eric recently demonstrated this with his Orion shot. Where he used a varying amounts of exposures to get much better image showing all aspects of the Orion nebulae. A long exposure may give you lots of nebulosity but you may miss the trap stars. Shorter exposures will give you less nebulosity but help you gain the trap stars. The final image is a composition to show all facets of the Orion nebulae.
Regards
Fahim
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