Quote:
Originally Posted by bmitchell82
I was under the impression rick that with read noise you can partially null the effect by the use of bias which is a map of your read noise.
Flats null the effect of light drop off and optical defects
Darks map your Thermal noise
combinations of these help you sort out and make the cleanest possible data for your effort.
Shot noise as your calling it am i right of the assumption that its what we normally call colour noise? and hence the reason for taking many long exposures to average this out and smooth the end image.
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Brendan, I think a lot of people are confused about the difference between unwanted signal and noise. I know I used to be...
Unwanted signal includes things like sky glow, sensor bias voltage and dark current. These are things you don't want in your image, but they are not noise in the technical sense. Bias frames are used to remove the unwanted bias signal from your sensor. Dark frames are used to remove unwanted dark current. Neither removes noise. In fact bias and dark subtraction actually adds some noise! We minimize this noise by taking many bias and dark frames.
Noise is about uncertainty and sampling. Think about doing a series of one minute exposures of a dim object and looking at one specific pixel. Because photons behave probabilistically, you might capture 7 photons in the first exposure, 11 in the second, 13 in the third, 9 in the fourth, etc. This variation is shot noise. By averaging the value over many exposures you reduce the contribution of the noise and get a better quality image.
There is also noise associated with bias voltage and dark current. Read noise is noise generated in the sensor and camera electronics when you read an image. Even if you received exactly the same signal several times, the image you read out of the camera each time would be different due to these (hopefully small) random variations.
Quote:
Originally Posted by WingnutR32
I would think that the best result will come from exposing the region in concern for a length of time that will fill up the wells without topping out and then repeat for as many times as you want to 'smooth' out the image (I am sure there would probably be a limit to the amount of stacks used, but I cannot answer that).
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Sam, I mostly agree with that. You probably don't want to fill the wells completely because they may behave non-linearly when they start to get full, especially if the sensor is anti-blooming.
There isn't a limit on the number of exposures you can stack, but you will find you get diminishing returns. The improvement in signal to noise is proportional to the square root of the total exposure. To make your S/N twice as good you need to increase exposure by 4 times. To make it three times better you need 9 times the exposure, etc.
Quote:
Originally Posted by WingnutR32
But the direct answer to the examples given would still be more detail in a longer exposure (assuming no stack) with lower signal to noise ratio (again, depending on hardware) and less detail and higher signal to noise ratio (smoothness) with shorter exposures with many stacks.
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Still don't agree with this
Quote:
Originally Posted by WingnutR32
As Brendan said though, many long exposures = WIN!
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Amen
