Median filter is a nonlinear digital filter used to remove noise from the guider frame. 

This noise reduction is applied while pre-processing the guider frame to improve the result of later processing. 

It preserves small stars, edges while removing impulse noise and hot pixels. By default, the median filter is set to 2x2 pixels. To disable median filter, you must enter 0 or 1. We do not recommend to disable the median filter.

In most cases, the default 2x2 pixels works well. The larger the window size, the more the impulse noise is removed, with large sizes, blurring increases and processing time increases.

By default SkyGuard uses a 2x2 pixel median filter for removing hot pixels and image local artifacts. If the mount can be moved by SkyGuard (with manual move for instance) but it does not register any motion this is usually a strong indication that hot pixels may overwhelm any guider frame actual astronomical patterns.

Since hot pixels (and other local image artifacts) do not move SkyGuard may fail to detect any image motion.
The median filter 2x2 default setting should be enough to deal with hot pixels, if not it can be increased, larger values lead to slower frame processing.

Using only the simple dark frame subtraction for removing the hot pixel may fail if the guider camera temperature in not controlled.

We recommend using the median filter, set at least at 2.