andrea tasselli:
Not so long ago there was a rather long discussion in AB about the merit and demerits of short exposures for DSO imagery with one folk clocking an amazing amount of short exposures for both M81 and M51, and one of the question that come up was how different are they if both have the same integrated duration? Off of happenstance (I forgot to turn off auto-saving in NINA preview) I collected a rather large amount of 5s exposures; to be exact 112 (once I pruned the worst ones, FWHM-wise) and since I did follow up with an actual imaging plan I can compare the results (same everything, one set immediately shot after the other). The imaging (regular) run takes 3 min integrations so 3 integrations are roughly equivalent to 112 5s (560s vs 3x180=540s) and here are the results:
It easy to guess which one is which (or just read the header) and this proof positive that very short exposures are a very bad idea for DSO. Not only the results are way nosier (112xRON vs 3xRON) but the overall depth is worse. This is with an IMX533 sensor so a low-noise, high efficiency modern CMOS.
I always love seeing visual comparisons to go along with the theory.
In this case, its a pretty raw and direct apples to apples, and I think that apples to apples comparisons are too often shirked in favor of apples to oranges when oranges better suits one of the approaches. ;)
I think, though, that there may not be consistency in what "short" vs "long" exposures are... In this case, its 5 seconds vs. 180 seconds. There are two things there...one, the difference is DRAMATIC, so the more dramatic difference in the visual results should be expected to a degree. The other, is that at 5s, its unlikely you are swamping the read noise to even a reasonable degree, and that's probably the key impacting factor here.
For me, 3m subs are not particularly long. Maybe not particularly short, either. Maybe I am just old school, but when I think of "long" exposures, I think back to when I was doing 10 minute subs, or even the 15, 20, 30 minute subs that a lot of the CCD imagers used when I first got into the hobby years ago. Back then, 5 minutes was "short" and anything under 60 seconds was just read noise, basically.
Now, 5 seconds, interestingly, is actually viable thanks to the very low read noise CMOS cameras have these days. That said, I think that such short exposures are probably better paired with a reasonably appropriate high gain setting, to minimize the potential read noise. In your case, half an electron difference between the gain you used, and the minimum possible gain, may not seem like much, but when your only getting a handful of electrons in each sub, that half an electron could make a difference (dynamic range notwithstanding.)
It would be interesting to see, though, when "very short" exposures become viable, and at what level of read noise and DR. Your 3 minute subs look to be thouroughly swamping the read noise. More than is necessary, probably, given the quality of the background signal there. At what point do the gains of "longer" exposures fall off? Is it 3 minutes, or 2, or 1m30s? At what point do "short" exposures start to look like longer exposures...10s, 30s, 60s?
A 5s exposure isn't just short, its very short. And even with CMOS cameras, unless your LP is psychotically bright, its doubtful that such short exposures would swamp the read noise to a reasonable degree. That doesn't mean, though, that some form of short exposure wouldn't be viable. There can be benefits to acquiring lots of shorter subs. For one, it allows much finer-grained culling of less than ideal or optimal subs, to optimize the final stack for some particular characteristic (i.e. star roundness, or detail sharpness, etc.) This kind of stack optimization is less viable with longer exposures (barring exceptional equipment like absolutely encoded mounts and the like.)
Anyway, I love a good visual example! Thanks for sharing.
