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Explanation please.


Jeff RV
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If i want to image a subject that say needs a 10 min exposure with a ccd imager to fully resolve, but for arguments sake i have to take in 10x1 min exposures (passing jumbo jets etc), or 2,3, min or even longer but below ten Min's.

How then when i stack them can it make a fully resolved image when none of the original exposures contain the full luminosity of the true image? if you get my meaning. :)

Does this mean that any image made up of an underexposed stack is technically a lie because the rest is made up by tweaking, This puzzles me!!

Please don't all give me an ear bashing at the same time, and remember you'll probably get banned for swearing on the forum. :)

Jeff.

Edited by Jeff RV
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Coming from a background of using Film for twenty years and then getting into CCD stuff I had the same problem of understanding...

The answer is that although each frame may be "underexposed" there's always one or two photons which have registered above the background noise; adding all these images combines these "odd" photons to give you a usable image. It's not like film which needed a minimum exposure to get each silver crystal to become "developable"....

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Jeff, you are correct in your assumption, if the CCD has not captured enough photons to register on the ccd then no matter how many exposures you take you will never capture that bit of data.

Taking multiple images reinforces signal and averages out noise. As noise is random taking multiple exposures will flatten (for want of a better or correct word) out the noise whilst as the true signal (Nebula/star etc) will always be in the same spot this gets reinforced.

Hope that makes some sort of sense! Basically taking multiple exposures and stacking will increase the signal to noise ratio and this has the benefit that you can 'pull' a lot more of the hidden detail out. BUT as I said above if it aint registered on the CCD due tot he exposure being to short then you wont be able to pull it out!!

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Thanks for the clarity on this lads, now i can understand it a little easier.

You see i know how Silver Halide works in a neg, and that's what was throwing me thinking them the same as data, as 10 negs all with just 10% exposure supposibly stacked would still only give you a 10% resolve. :) :) Complicated!!

Cheers.

Jeff.

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