See Telescope coordinate systems looking towards sky (excludes Nasmyth foci) , See Telescope coordinate systems looking towards sky (Left Nasmyth focus) and See Telescope coordinate systems looking towards sky (Right Nasmyth focus) can be used to write down formulae to be used for the following purposes (these are not the only possibilities; they are those needed by the DCS during normal operation):
Subsequent sections describe each of these in some detail and specify which DCS keywords correspond to the various angles and modes.