Sometimes it is useful to shrink images. The Binning command does this in the same manner as binning inside the camera – simply combine the adjacent pixels together into a single ”super-pixel”. Unlike a CCD camera, this function averages the values instead of summing them; however the effect is otherwise identical. (Binning inside the camera may reduce total read noise, though, if the binning is done "on chip").
Simple binning does not ensure that the result meets the Nyquist Sampling Criterion. This means that small point sources like stars can all but disappear. The correct way to resize an image is to first low-pass filter it, so that no spatial frequencies exceed one half the new sample interval. This prevents the addition of aliasing distortion into the image. The Half Size command includes such a Nyquist filter.