I tried photographing moon and Saturn with a 8'' Dobsonian (without tracking). And it was a breeze. But I had to follow the '600 rule', i.e I only exposed it for less than (600/f) sec ~ 1/2 sec. But now I want to photograph deep sky objects, and since there is no tracking the most I can do is give exposures of less than one second. Would these small exposures be good for stacking? What should be the ISO ? What else can I do to take better deep sky shots.?
Answer
There is no substitute for per-image SNR (Signal to Noise Ratio). A half-second exposure following the 600 rule won't be enough. You have to factor in read noise as well as photon shot noise. Photon shot noise can be delt with via stacking, but read noise...read noise diminishes detail and in the deeper shadows, can eliminate it entirely. With half-second exposures, your overall SNR is going to be so low as to not be worth it. You could stack 1000 frames with half-second exposures, and it wouldn't come anywhere close to stacking 10 frames with much longer exposures.
Also, keep in mind, increasing ISO does not actually increase sensitivity. The interchanged use of "sensitivity" and "ISO" has lead to a grave misunderstanding of what ISO really is. Increasing ISO does not actually improve your SNR, it simply amplifies a lower signal by a certain factor. (Increasing ISO does marginally improve IQ, by amplifying the signal before read noise is added, but it is still quite marginal overall.)
The only way to improve the end results is to gather an overall image signal that is sufficiently strong enough to make read noise a relatively inconsequential factor. You still want to stack, as it is difficult to maximize SNR even with longer exposures, and photon shot noise will always be a problem that can be minimized with stacking. To that end...either using a camera with bigger pixels that have a naturally higher SNR, or using a tracking mount, are really the only ways to improve your deep sky shots.
IT'S ALL ABOUT SIGNAL TO NOISE RATIO!
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