The star trails in the sky can be seriously modified by atmospheric refraction depending on their positions, mainly close to the horizon. This is why it can be necessary to change the tracking speed depending on the altitude of the scope position. As a kind of rule of thumb, please apply the settings below (mainly in case of longer exposures and longer focal distances):

1. Use Sidereal tracking speed when the telescope is pointed close to Zenith;

2. Use average King rate tracking speed in the mid-ranges of altitude;

3. Use auto-guider simultaneously with TDM (using auto-guider input of TDM) if the telescope is pointed very close to the horizon.

However, the difference is not to much: 24.6sec/day, but can be remarkable during longer exposures.

Important: King's approach is applicable only if the RA shaft of the mount is pointed onto the refracted pole but not onto the true pole.

Regarding the detailed explanation in difference between Sidereal and average King rate tracking speed please find the relevant pages of Edward Skinner King's book below issued in 1931 by Eastern Science Supply Co., Boston.