Quote:
Originally Posted by Peter Ward
This is interesting as it may show a mis-understood element others may have about signal, noise and stacking.
Stacking only works if you are adding signal to an image at a rate faster than you are adding any noise. As a result using the same image data its very much a case of if you divide the same number by itself, you get one, no matter how big the number.
Similarly if the image has a high noise level, through say a short exposure of say a DSLR of 30 seconds or less, you are still not building up signal very quickly. Akin to dividing a number by a little less than itself...sure more than one, but not a lot more.
Stacking works very well when the signal swamps the noise (eg, 20 minute rather than 20 second exposures).
As for the Sigma 300mm F2.8. It's not bad. but needs to be stopped to about F4 and you also need to drop a UV+IR block filter in the filter bay.
Hope that helps
Peter
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To put this into perspective.
1 The signal to noise ratio for a number of summed images is equal to the sqare root of the number of images multiplied by the mean photon flux.
2 If you compare the snr of a single image to the snr of a summed image, then the the summing improves the SNR by a factor of the square root of the number of images.
3 So if you want a four times improvement over the single image then you need to sum 16 images.
I think this would apply regardless of whether the exposure times are 30 seconds or 30 hours.
Regards
Steve
