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Shooting under suburban skies in LRGB, which most of us do, you will find that your exposure times are limited by the inherent sky glow of your location. Under dark skies, your subs can go as long as your ability to track will allow, but under light polluted skies rarely longer than 10 minutes (and usually much less). Using narrowband filters helps here and you can go much longer.
There's a calculator on the Starizona website for calculating optimal sub-exposure times, I usually double it (just to be on the safe side). There comes a time when you are just accumulating more signal and noise equally for dim objects. In fact I'd say that if you need 10 minutes subs to see something dim, then you're pushing the proverbial uphill to get a decent image from the 'burbs.
I'm taking a picture of the Antennae Galaxies at present, I love the faint wispy tails, but boy are they dim. Have a look at a 10 minute sub, then a stack of two hours...
Take no notice that I need to redo my flats, but look at the antennae. In PS3 for the stacked image there is about 4% difference in the k value about halfway along the tails, this is about the same as in the single sub. In CCDStack the mean value for a small section of tail around halfway along is 1233, the mean for the background next to that is 1226. For the stacked image the values are 1237 and 1228, I'm really up against it with this target...
These are 10 minute exposures. I'm going to try to exposre for the same time in 5 minute exposures next time we get clear skies, maybe even add in some 15's just to complete the experiment. The final image won't be anything to write home about, but it'll at least satisfy my inquiring mind about sub length. If I can get the same result with more 5 minute exposures then I will do it that way.
Oh, dithering is a good idea, but sometimes it's impractical. As I use an AO unit, I can't always dither, as, like in this image, the guide star is placed at the top edge of the guide chip, automated dithering would almost certainly move it off the guide chip. I use electronic methods for hot pixel removal. I'm always left with a couple, but CCDStack is excellent and removes most of the artifacts, once you learn how to use it.
Cheers
Stuart
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