What's the difference?

[fatwoul]

OK, someone please tell me off for asking what I know is already here - I've seen it but I can't remember the explanation given:

What is the difference between an exposure of (say) 10 minutes, and 10 exposures of 1 minute? Why is the end result different, even if the total exposure time is the same?

Someone was just asking me, and I was getting into a pickle trying to answer. I'm pretty sure it's explained in MEPC, but I can't find the page.

[brianb]

10 x 1 min exposures contains sqrt(10) = approx. 3.1 times as much reasout noise.

1 x 10 min exoosure burns out the detail in bright parts of the image at 1/10 the surface brightness that 10 x 1 min exposures do.

[fatwoul]

Thank you, Brian. That's the nice, concise, simple explanation I was hoping for. I would very much like to quote it verbatim and take the credit...

...but they've already signed out of MSN. My plagiarism will have to wait until tomorrow.

[dmahon]

I thought more exposures and stacking reduces the noise in the image, but it won't show as much detail?

[FraserClarke]

I thought more exposures and stacking reduces the noise in the image, but it won't show as much detail?

If you are comparing exposures of the same individual exposure time, that is true (i.e. 10 x 1 min exposures will give better signal-to-noise than 5 x 1 min exposures). It's not intrinsic to the *number* of exposures though, it's linked to the total exposure time.

Brian's explanation is brilliantly concise

[dph1nm]

I thought more exposures and stacking reduces the noise in the image, but it won't show as much detail?

The best way of thinking of this is that more exposure and/or srtacking increases the signal-to-noise ratio in the final image, which is the desired aim.

Whether the measured value of the noise increases depends on whether you decided to average by the number of stacks at the end or not. I have noticed that this tends to cause some confusion ...

NigelM