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DPanzer17 t1_ja73gl9 wrote

Reply to comment by KingRandomGuy in I shot over 3600 one-second exposures to get my sharpest image of a galaxy to date by J3RRYLIKESCHEESE

I'd love to try too, there's just one thing unclear for me. Why 1 second exposure? Wouldn't 10 seconds exposures have allowed him to take 1/10th of the photos to get approximately the same result?

But I guess long exposures are not always a good idea, I tried 30 sec and the result wasn't satisfying (but I might have got the focus wrong there)

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KingRandomGuy t1_ja74wh7 wrote

So the downside to long shutter speeds is that if any motion happens during the duration of the exposure, then you will get blur in your image. This is why photos of people in lowlight often aren't sharp - people moving a little bit during the long exposure makes them have visible blur.

For astrophotography, the problem is that the sky is slowly rotating. This is barely perceptible to us, but it's enough rotation that a 10 second exposure may result in star trails - basically stars will appear like streaks instead of points.

So, as you've correctly identified, we want to maximize our exposure time, but you can't go too long or you'll get blurry images. This can be alleviated by star trackers, but that's an extra piece of equipment. One rule that can help with this is the 500 rule, which states that anything over 500/f seconds, where f is your lens's focal length (full frame equivalent), will cause motion blur. Do note that you'll often need to be well under this number to avoid blur, but it's a nice rule of thumb.

Clear skies!

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Total-Oil2289 t1_ja7api5 wrote

What's the difference between this and so-called "lucky imaging" where my understanding is that you're taking lots of short exposures and keeping the ones that are least affected by atmospheric turbulence? Is it just the same solution for different problems?

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KingRandomGuy t1_ja82qp1 wrote

It's a similar solution to lucky imaging, but lucky imaging specifically requires that your exposures are short. You can still stack very long exposures for deep sky objects and get a great result (assuming you are tracking).

The concept is similar though; in both cases you are stacking to increase the signal to noise ratio, and you should throw out bad frames.

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