GNU Astronomy Utilities


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8.2.1.2 Instrumental noise

While taking images with a camera, a dark current is fed to the pixels, the variation of the value of this dark current over the pixels, also adds to the final image noise. Another source of noise is the readout noise that is produced by the electronics in the CCD that attempt to digitize the voltage produced by the photo-electrons in the analog to digital converter. In deep extra-galactic studies these sources of noise are not as significant as the noise of the background sky. Let \(C\) represent the combined standard deviation of all these sources of noise. If only this source of noise is present, the noised pixel value would be a random value chosen from a Gaussian distribution with

$$\mu=I_{nn}, \quad \sigma=\sqrt{C^2+I_{nn}}$$

This type of noise is completely independent of the type of objects being studied, it is completely determined by the instrument. So the flux scale (and not magnitude scale) is most commonly used for this type of noise. In practice, this value is usually reported in ADUs not flux or electron counts. The gain value of the device can be used to convert between these two, see Flux Brightness and magnitude.