Dithering

Dithering is an important part of the modern image acquisition process. CMOS and CCD sensors suffer from various types of electronic noise, such as fixed-pattern noise, and defects such hot and cold pixels. Photons of light from stellar objects fall onto the sensor and will effectively be lost if the photon falls on a dead or hot pixel. The act of dithering is a commanded move of the mount between successive exposures that very slightly alters, on the pixel scale, where photons fall on the sensor. This means that one exposure can capture light (ie, data) from an object where that same light fell on a dead or hot pixel in the previous exposure. Image alignment issues are easily handled by most astronomy stacking and image post-processing applications.
The different positions of single light frames will make it more effective for image integration algorithm that implement an outlier rejection method (like sigma-clipping) to reject fixed-pattern noise as well as hot and cold pixels. This is because hot and cold pixels are always in the same position in the camera sensor and dither will make them appear in different places for each frame after a dither move.

Because dithering is an operation that must be coordinated with guiding (remember, the mount is being purposefully moved by a few pixels, which guiding would reflexively attempt to counteract), the dithering operation is managed by the guiding application such as PHD2.

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