Yes, one of my favorite Deep Sky Objects…the Ring Nebula, M57 is again front and center. Or was it – overhead? This is the first DSO I ever saw back around 2014 through my 4.5″ Newtonian. It was no more than a faint smudge then. Now in the 8″ CPC-800 it’s a brighter fuzzy doughnut but still benefits from long exposure photography. It’s a go-to object during “impromptu stargazing” outings. People always enjoy seeing the ring.
My M57 shot in June was an experiment to see how short of a sub-exposure could be useful using my Canon T5i DSLR if I stacked a bunch of them. Yes, I’m still using that DSLR camera. No fancy astro camera yet, even though I understand the benefits of having one. I’m not ready to spend the bucks for a camera with only one purpose, whereas my DSLR is so damned convenient and all-round useful!
Anyway, conventional wisdom suggests stacking many shorter exposures will yield better results than fewer (or a single) long exposures. Stacking images means to perfectly align them, stack them like pancakes, and then average each individual pixel like sticking a toothpick through the stack. The averaging operation causes noise to drop out of the image.
But there are limits and like anything else, consider the extremes. 300, 1-second sub-exposures or a single, 5-minute exposure – which is best? I suspect the 1-second subs just won’t cut it on my f10 focal ratio setup that is starved for light. That telescope is a pretty “slow” 2000 mm lens! And a 5-minute exposure requires careful tracking and good luck that a Starlink satellite won’t ruin the image. Something in the middle, perhaps? 30 seconds?
The previous M57 was shot by stacking around 80 images at 30-seconds each at 1600 ISO. 30 seconds still seems short to me, and the subs were pretty noisy. But I wanted to see what stacking 80 images would do. And I do think that there was generally more detail and less noise than in my single exposures. But I lost much of the red channel. The red channel of my camera is weaker than the other two, and quite noisy at 30 seconds per image. Even with stacking and stretching, I just couldn’t bring out the red perimeter of M57 very well:
Here’s a shot from 7-7-22 using 34, 90-second subs. I shot 38 subs but had to throw out 4 shots with satellite trails:
I do like the second image better and upon close examination, it has better detail. Although to be fair – the second is about 20% longer total exposure time than the previous image. That’s just the way it worked out. But it was apparent in the two sets of images that the 30 second exposures just didn’t have much signal in the red channel. I had lots of trouble reducing the red channel noise in the first stack, whereas the second stack gave me considerably more signal to work with. 90 seconds seems a pretty reasonable sub-exposure time for my f10 setup.
This is probably the best image of M57 that I’ve got from my Canon DSLR. I even stretched out that little galaxy – IC-1296 in the background. It’s 235 million light-years away!
Oh, and just for reference, check out the Hubble version! I’m not quite there yet!