TypesofnoiseinaCCD.docVIP

Types of noise in a CCD Basics When a CCD image is taken, noise will appear as well as the main CCD image. Noise can be thought of as unwanted signal which doesnt improve the quality of the image. In fact, it will degrade it. The main problem with noise is that most noise is essentially random, and so cannot be completely removed from the image. For example, if we know that a noise source contributes 10 units on each image we can subtract those 10 units from the image. If we only know that the noise is around 10 units, then we cant completely remove all of this noise (as we dont know its exact value). The main contributions to CCD noise are: Noise on the image itself (shot noise) The detection of photons by the CCD is a statistical process. If images are taken over several (equal) time periods, then the intensity (the number of photons recorded) will not be the same for each image but will vary slightly. If enough images are taken, it will be seen that the deviation in intensity found for each image follows the well known Poisson distribution.? In effect, we cannot be sure that the intensity we have measured in a particular image represents the true intensity as we know that this value will deviate from the average. It is this deviation which is considered to be the noise associated with the image. As the deviation is known to follow a Poisson distribution, we know that the likely deviation will be plus or minus the square root of the signal intensity measured. Thus, if we measure a signal intensity of one hundred photons, then the noise on this signal will be ten photons. If we measure a signal intensity of one thousand photons in the image, then the noise on this signal will be about thirty one photons. Thermally generated noise: Additional electrons will be generated within the CCD not by the absorption of photons (i.e the signal) but by physical processes within the CCD itself. The number of electrons generated in a second will be dependent on the operatin