I have to disagree with this. If the noise is truly random, i.e. it follows a Poisson distribution, dithering will not help at all. Truly random noise would vary from shot-to-shot in exactly the same way that a random dither would. Random is random. There would be no reduction, or increase, in random noise nor a change in SNR due to dithering.
Dithering serves a single purpose - it moves fixed-pattern noise around the field so that outlier pixels can be corrected during stacking with sigma-clipping rejection. Fixed-pattern noise is primarily warm and hot pixels but can also include column defects and some (but not all) banding from readout. The biggest benefit that a lot of people donât recognize from dithering is that it can, in some cases, negate the need for darks. Darks largely serve the same purpose as dithering in that they are intended to provide a means to subtract fixed-pattern noise from the subs before stacking. However, unlike sigma-clipping rejection, which adds no noise to the final image, darks subtraction does add noise. In this case, by ânoiseâ, I mean random variation in pixel brightness that follows a Poisson distribution. All of the noise in the dark images adds to the noise in the lights in quadrature. So, calibrating with darks always adds noise to the final image. Thatâs the reason that we collect and stack many darks. The stacking process tends to even out the random variation in pixel brightness by averaging them, making the amount of noise added to the final image less.
Since dithering adds no noise to the final image and is (typically) effective at preparing the image stack for fixed-pattern noise removal using sigma-clipping, it is possible, and even preferable to use dithering in place of dark subtraction, which is what I do. Of course, for this to work, your camera needs to add no large-scale fixed-pattern noise (such as amp glow) to the party since only pixel-level fixed-pattern noise can be removed using dithering and sigma-clipping.
Dithering can also be extremely helpful for DSLR users. Warm and hot pixel brightness varies with sensor temperature - the warmer the sensor, the more quickly warm and hot pixels get brighter. Ambient temperatures may change by tens of degrees over the course of an imaging session and as a result, hot pixels in some images may be several times brighter than hot pixels in others. Dark frame subtraction will subtract a fixed hot pixel brightness from each frame, and for those frames with differing hot pixel brightness, the subtraction may be too much, or too little. Dithering adds a secondary method of correcting remaining hot (or cold) pixels by again moving them around the frame to allow sigma-clipping to detect them.
Sorry⊠I went on longer than I intended toâŠ
Tim