Probably a stupid question...

[Superewza]

but, what people seem to do is take many exposures and stack them. My question being - what 'extra' information is going to be captured by using more than one exposure? As far as i can tell if the subject is the same, the camera set up is the same and pretty much everything about the photo is the same or very similar for all the exposures... why not just duplicate a single exposure and stack that?

Thanks

[Demonperformer]

The advantage of stacking lots of subs is that the "noise" in the subs, which is fairly random, will be different in each sub and the program can remove it. If you used just one sub, the noise would be the same in each version and so the prog could not identify it to remove it.

Think that's it, anyway.

BTW: for future reference, there is no such thing as a stupid question:)

[Mark-mck]

i belive it also has something to do with "seeing", each picture will be slightly different due to atmospheric disturbance, i dare say the difference will be hardly notacable to the naked eye, but get 30 or more subs and a lot more detail will come out.

well thats what i've gathered from the research i've been doing before taking the leap into AP myself, hopefully someone alot more learned than myself will be able to confirm that or tell me i'm a total idiot lol

[Luke]

There's a really nice example of how stacking can reduce noise here:

"The image is a single exposure, then stacks of 5,10, 20 & 35 exposures.":

http://www.middlehillobservatory.co.uk/articles-primers/image%20processing%20pics/M94-GIF.gif

and the article is at:

pic and description page

PS Many thanks Rob for the primers!

[carastro]

to do with "seeing", each picture will be slightly different due to atmospheric disturbance

that's the main reason, so stacking lots of slightly different images together adds up to a crisper and more detailed image.

Also helps to reduce the noise in the same way.

Carole

[Demonperformer]

Thanks for posting the link, Luke. Really shows the difference it makes.

[dph1nm]

i belive it also has something to do with "seeing", each picture will be slightly different due to atmospheric disturbance

Not really. The variations are intrinsic to the object, so you would get the same improvement from space.

Basically, the uncertainly in the number of photons (often called the shot noise) you get from an object in a given time is proportional to the square root of that number (this is all down to quantum mechanics). Now for a 'nice' image you need a higher ratio between the signal and the noise (the signal-to-noise ratio, or SNR). A bit of maths show you that the more photons you get, the higher this ratio (N/sqrt(N)) becomes. So anything that gets you more photons is good. This can be a longer single exposure, or many small ones stacked together (or a bigger scope!).

NigelM

[themos]

It's because of quantum mechanics.

No, really.

We are capturing so little light that the discrete nature of quantum reality becomes apparent. In daylight we don't have that problem because there's so many photons coming in that we need to close the shutter quickly lest we get too many and our image whites-out. Daylight is like a river. Dark sky is like a dripping tap.