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throughawaeladdie t1_ja6bbvq 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

so if op had chosen to do 2 second exposures instead, the pre-stacked photos would have more noise right?

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KingRandomGuy t1_ja6fed5 wrote

No, they would have less noise. A longer exposure means that more light hits the sensor, so you get more signal for the same amount of noise. In this case, assuming that the ISO (sensitivity) of the camera remained constant, doubling the exposure time would double the brightness (signal) while having the same noise level. In turn, the pre-stacked images would have a higher signal to noise ratio.

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throughawaeladdie t1_ja6fkcp wrote

ah i see where i went wrong. thanks for explaining this to me!

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KingRandomGuy t1_ja6fsxk wrote

No problem! I'm happy to answer any other questions too.

Astrophotography is quite a technical hobby but it's really rewarding IMO. I'd recommend giving it a try if you have the time and resources.

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DPanzer17 t1_ja73gl9 wrote

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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