Thread: Image stacking for deep space

1. Image stacking for deep space

I know how image stacking helps reducing the signal to noise relation, and that is nice for planetary imaging. However, I still don't understand how it helps long exposition images for deep space. Do they increase the effective exposition of the resulting image?

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One can think of it as increasing the exposure of the resulting image but it's not quite accurate. Stacking works since adding several signals (in this cases 2D digital signals or images) with Poisson distributed noise statistically results in that the signal part scales with the number of stacked images but the noise only increases with the square root of the number of stacked images. In a sense the random noise sources partly cancel out each other and we get a better signal to noise ratio.

In your astroimage you got three different signals, photon signal (the true image we want), dark current and bias/offset and each signal has its own independent Poisson noise. The calibration with darks and bias removes the dark current signal and bias signal from the image and we are left with only the photon signal however we still have all the original photon-, dark current- and bias noise + the new noise introduced by the dark and bias calibration frames.

ETA:
Forgot something.

This signal/noise behavior is the also same reason why a long exposure is better than a short. The only difference after stacking between ten 1 min exposures and one 10 min is that the short exposures have ten independent bias noises instead of one. Dark current and photon signal will be the same in both images. Since bias noise often is quite small one can usually treat stacking as if it just increased the exposure time.
Last edited by glappkaeft; 2012-Apr-21 at 11:31 AM.

3. Hi,

If an object is so dim it only reaches the first coulpe of shades of gray (in a scale of, say, 0 to 255) in a single exposition, is it possible to use image stacking to reproduce the image not just brighter, but with more shades in it?

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Yes but if it's the object or part of the object is too weak then longer individual exposures generaly work better. If the photon signal is weak the (constant per image) bias signal and bias noise are too large compared to photon/dark current signal/noise.

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