 This flat mirror is placed on the altitude axis, so that the beam exits through a hole in the middle of the altitude bearing.This means the eyepiece or instrument does not need to move up and down with the telescope as the tertiary mirror's angle with the main telescope axis is adjustable as a function of the telescope's pointing and the star's elevation above the horizon;
therefore a heavy instrument can be used without upsetting the balance of the telescope or increasing the load on the altitude bearings.This has significant advantages for spectrographs and other heavy instruments typically used at research observatories.