Thought bubble - stacking

[PeterEde]

Why do we need shot after shot of the same thing. Why can't we simply create a dozen copies of the same image and stack that? No issue with alignment. Minimise noise. What do we get from hours of shooting the same image we can't get from hours worth of the same image?

[cometcatcher]

SNR - signal to noise ratio. While there are many forms of noise, the camera produces something that looks like film grain, or "snow" from an old analogue TV. Every different sub we take has the same signal but the noise changes. If we average lots of frames we are left with signal (the pic we want) while the noise is subdued from the averaging process.

Stacking the same frame would stack the same noise as well as the same signal so there is no gain in SNR. That's why I bought the K-5, it has better SNR than my old K-x.

[PeterEde]

Ok that's sounds reasonable. Plus it gets us out the house longer. Good for some not so much for other :o

[raymo]

If you have a $15 intervalometer, it doesn't even keep you out of the house. As far as SWMBO is concerned, it's my most valuable piece of equipment.
raymo

[MrB]

As Kevin has said the noise in each exposure is different and can therefore be cancelled out.
Pixinsight has a great little example of the process at the following page, ignore the math and blurb if you wish but scroll down to Figure 2 and check the links to the right of the example image.
This shows the effect nicely.
http://pixinsight.com/doc/tools/ImageIntegration/ImageIntegration.html

Edit: it might take some time for the images to load, their site seems to be very slow.

[rustigsmed]

http://deepskystacker.free.fr/english/index.html

this gives a good indication by hovering your mouse over the different number of subs.

if you have a canon you can download magic lantern which has an inbuilt intervalometer http://www.magiclantern.fm/features.html free

[PeterEde]

I have tried ML b4. Found it confusing
Looks like I should give it another look see

[RickS]

Basically correct but worth noting that while one source of noise is the camera (read noise, dark current noise) there's also a completely unavoidable and inherent source of noise called shot noise. Even if you had a perfect, noiseless camera there is still uncertainty in the measurements of photons coming from your target object. If your subs are long enough (may only hold for a cooled CCD where dark current and the associated noise is controlled) then this is the major source of the noise you can see in individual subs and why we take multiple subs.