Quote:
Originally Posted by Slawomir
To my understanding, the darker the skies, the cleaner the camera and the brighter the object the shorter the subs to get a good SNR. And inversely, with light polluted skies and with a noisy camera and when imaging a dim object exposures need to be much longer to achieve the same SNR. Or am I missing something obvious?
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Suavi,
To restate what you said more correctly: the darker the skies, the cleaner the camera and the brighter the object the shorter the
total exposure time needed to get good SNR.
The effect of dark skies on sub length is the tricky bit. Because they are dark there is little sky glow and hence little sky glow noise (which is the sqrt of the number of photons detected). That means that read noise makes a much bigger contribution to the total noise and long subs are needed to overcome this. This seems counter intuitive but it makes sense if you think about it.
Note that the SNR and quality of a sky limited sub taken under dark skies will be much, much better than a sky limited sub taken under bright skies. The latter sub will contain a lot of noise associated with the sky glow (as well as the unwanted sky glow signal itself.) Despite needing long subs under dark skies, the total exposure time required to get acceptable overall SNR will be much shorter.
Cheers,
Rick.