Can you shoot deep-sky objects with an 8-inch Dobsonian and no tracking?
Asked 4/30/2013
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I can photograph the Moon and Saturn with my 8-inch Dobsonian, but for deep-sky objects I’m limited to very short exposures because the telescope is not tracking. If my exposures are under about a second, is it still worth stacking them for deep-sky imaging? What ISO range is reasonable, and are there any practical tips for getting the best results from a non-tracking setup?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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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!
Originally by user124. Source · Licensed CC BY-SA 4.0
user124
13y ago
0
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You can stack very short exposures, and software such as DeepSkyStacker can align and combine them, but a non-tracking Dobsonian is a major limitation for deep-sky work.
The key issue is signal-to-noise ratio: very short sub-exposures collect little light, so each frame has weak signal and read noise becomes a big problem. Stacking helps reduce random noise, but it cannot fully make up for extremely low-quality individual frames. In practice, hundreds or even thousands of short exposures may still fall well short of a much smaller number of longer tracked exposures.
From the shared experience here, workable settings on a non-tracking 8-inch reflector were around 1/10 second or shorter, ISO roughly 1000–1600, and shooting in RAW with 100–250 frames minimum, ideally many more. Keeping the target near the center of the frame can help stacking software register the images more easily.
Also, raising ISO does not increase the sensor’s true sensitivity; it mainly changes gain, so don’t expect ISO alone to solve the problem.
Bottom line: yes, short-exposure stacking can produce modest results, but for significantly better deep-sky images, adding tracking is the real upgrade.
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