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danielbb

Stacking vs long exposure

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Im confused, i am new to astrophotography and my first target has been andromeda, i have done the following.

1 x 8 minute exposure at 1600iso

and

10 x 7 minute exposure at 800iso plus 3 darks.

The single higher iso exposure beats the hell out of 10 stacked no matter how much i play with it. I dont understand why :- It seems the only way i can improve my final image is increase exposure time. Stacking just seems like i get the original image 10 times with no incremental info added on each stack. I have tried maxim and deepsky stacker same thing.

Maybe i'm missing something obvious here? Any ideas? Or does a lower ISO need a huge amount more exposure time to match?

I'm using 550d ed80 with CLS filter. Autoguided.

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The 8 min exposure at 1600 iso is recording a lot more light than the individual 7 minute 800 iso images. Combining the 7 reduces the noise in those ones but it makes sense to me that the 8 minute exposure would give a better response. Stacking a bunch of them should give a good result.:)

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In short, the longer the individual shot, the more faint stuff you can get. The more shots you get (long or short) for stacking, the lower the noise will be.

noisereductionOIIIc.jpg

For example I recently did the above. The left is a stack of 61 images, the right is a stack of 15 images. The single shot was a right mess!

Going back to the original question, *if* the details are not too deep in the noise floor, I wouldn't expect there to be too big a difference between the 8 or 7 minute shots even with the ISO difference. But stacking the 7 shots should give a lower noise output. I don't know how either Maxim or DSS works as I don't use them, but at the end you do need to adjust the output to bring the levels up. If you don't do that, the ISO1600 one would appear brighter.

Beyond that it is hard to advise without playing with the raw data.

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I use DSS, how did you go about manipulating the image once stacked, i.e. did you use levels and curves to bring out the detail. If you shot in RAW (which you need to do) there should be 2 to the power of 16 (2^16) shades to tease out of the image by stretching the histogram. I say this because I know how bad an image looks at first when its just finished stacking in DSS:)

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Not many DSLR's actually ouput true 16 bit raws usually they are 12 or 14 bit thats zero padded out to 16 bits... the result is that the "data" spike gets offset...

Peter...

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The 10x7 minute stack should have been far less noisy, allowing you to stretch the curves.

Here's another example of the effects of stacking, taking from my recent Heart Nebula:

gallery_5051_1080_1075975.gif

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The 8 min exposure at 1600 iso is recording a lot more light than the individual 7 minute 800 iso images

Not really - ISO doesn't affect the amount of light detected, so the difference between the 8min and 7min subs will only be 1.14x (8/7) which will hardly be noticeable. As others have said, once you stack the 7min subs the result should be much deeper (better signal-to-noise ratio).

NigelM

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Not many DSLR's actually ouput true 16 bit raws usually they are 12 or 14 bit thats zero padded out to 16 bits... the result is that the "data" spike gets offset...

Peter...

ah thanks 14 bit does ring a bell now you mention it:) it that case 2^14 shades to tease out in post processing:)

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