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  1. #1
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    Default Stacking 1 image multiple times vs multiple images 1 time



    Hi,
    I had a query. I haven't dabbled into astrophotography but wish to do it soon. There is this 1 question that is eating my brains. Now when images are stacked eg. say 20 x 1 min exposure, does it mean you have to take 20 "Different" 1 min exposure images and stack them, or is it that you take just 1 image with 1 min exposure and stack it 20 times? I might be wrong but I feel its pointless to spend 20 minutes when it can be done in 1 min (after all the view on the sensor wont change rite?)

  2. #2
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    Default

    You take 20 seperate images ( of ther same target ) say at 1min exposure, you would then stack all those 20 , say in Deep Sky Stacker, thus increasing the signal to noise ratio.

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  4. #3
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    Default

    If you stack the same image 20 times, you're just adding it to itself. You'll get exactly the same image out that you put in.

    If you stack 20 different images, the program uses the difference between the images to get more information about the image itself. E.g. camera noise - it varies a lot between images, so it's easy to spot and remove, while the actual object doesn't change much so it's easy to keep. There's a lot more to it, but that shows why you need multiple images.

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    If you want to get a better handle on understanding the relationship between stacking different images, try doing it manually. I am lucky I have PCS4 and so I can do it this way.
    It is a great way of seeing how the signal gets stronger in each image stacked. It is not as quick as DSS but it is a great way of learning...
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    Default

    Like psonice said, when stacking, the software compares all the images, "try" to eliminate what has change (hotpixels, atmospheric instabilities, clouds, mount movement, ...) and keep what has not change (the actual thing you are trying to photograph). To do this, you need multiple images of the same part of the sky.

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    Default

    The basic trick is actually very simple. An object (like a faint galaxy) is non-random and will not change significantly from one shot to another, and so stacking two shots of it makes it brighter since equal pixels add and become twice as 'strong'. Camera noise, on the other hand, is random and fluctuates between shots so that when you add them up, the noisy pixels average out. In the signal processing field, "stacking" is referred to as "averaging" and used to clean up noisy measurements.

    Like Dublin sky watch suggested, try doing it manually. That should illustrate the concept.
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    Thank You guys. Now I understand why you NEED to take multiple images. I didn't realize that stacking the same image would not help in eliminating the noise.

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    Default

    As mentioned, that would make the noise non-random and the stacking process would consider it part of the image information. There is a great explanation on the DSS site. And here is the page of FAQ's about the DSS program. There are several other programs of varying cost Maxim, Nebulosity, CCDsoft/Stack/Ops ... but the great thing about DSS is that it is free.

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