Telescope mounts allow the telescope to follow celestial objects as they traverse the sky.
On an altaz mount, which moves horizontally and vertically, the weight of the telescope sits immediately above the mount.
However, withan equatorial mount one of the axes – the right ascension axis – is tilted at an angle that matches your latitude.
This change in orientation places the weight of the telescope off to one side of the right ascension axis.
So to stop the telescope from rotating downwards under the effect of gravity, a counterweight placed on an extension bar is required to exactly balance the weight of the telescope tube.
Email your astronomy queries to contactus@skyatnightmagazine.com and they could be answered in a future issue of BBC Sky at Night Magazine.
