A method of calibrating a surveying instrument, wherein the surveying instrument comprises: a base; an alidade rotatable about a first axis relative to the base; and an optical measuring instrument having a measuring axis, wherein the optical measuring instrument is rotatable about a second axis relative to the alidade; wherein the method comprises: providing a beam path for a light beam using components including a light source for emitting the light beam, zero or more lenses for collimating the light beam, zero or more mirrors (50, 52) for folding the beam path, zero or more beam splitters, and a position-sensitive detector for detecting light of the light beam, wherein the light source is fixed to one of the base, the alidade and the optical measuring instrument, wherein the detector is fixed to one of the base, the alidade and the optical measuring instrument, wherein at least a portion of the beam path extends between one of the components, which is fixed to the base, and one of the components, which is fixed to the optical measuring instrument, and wherein the beam path exists for at least a first range of rotational positions of the optical measuring instrument about the second axis; performing plural measurements; and determining at least one error of the surveying instrument based on the plural measurements; wherein each measurement includes detecting, using the detector, light of the light beam traveling from the light source along the beam path to the detector when the alidade is in a given rotational position about the first axis and the optical instrument is in a given rotational position about the second axis. a base; an alidade rotatable about a first axis relative to the base; and an optical measuring instrument having a measuring axis, wherein the optical measuring instrument is rotatable about a second axis relative to the alidade; wherein the method comprises: providing a beam path for a light beam using components including a light source for emitting the light beam, zero or more lenses for collimating the light beam, zero or more mirrors (50, 52) for folding the beam path, zero or more beam splitters, and a position-sensitive detector for detecting light of the light beam, wherein the light source is fixed to one of the base, the alidade and the optical measuring instrument, wherein the detector is fixed to one of the base, the alidade and the optical measuring instrument, wherein at least a portion of the beam path extends between one of the components, which is fixed to the base, and one of the components, which is fixed to the optical measuring instrument, and wherein the beam path exists for at least a first range of rotational positions of the optical measuring instrument about the second axis; performing plural measurements; and determining at least one error of the surveying instrument based on the plural measurements; wherein each measurement includes detecting, using the detector, light of the light beam traveling from the light source along the beam path to the detector when the alidade is in a given rotational position about the first axis and the optical instrument is in a given rotational position about the second axis. providing a beam path for a light beam using components including a light source for emitting the light beam, zero or more lenses for collimating the light beam, zero or more mirrors (50, 52) for folding the beam path, zero or more beam splitters, and a position-sensitive detector for detecting light of the light beam, wherein the light source is fixed to one of the base, the alidade and the optical measuring instrument, wherein the detector is fixed to one of the base, the alidade and the optical measuring instrument, wherein at least a portion of the beam path extends between one of the components, which is fixed to the base, and one of the components, which is fixed to the optical measuring instrument, and wherein the beam path exists for at least a first range of rotational positions of the optical measuring instrument about the second axis; performing plural measurements; and determining at least one error of the surveying instrument based on the plural measurements; wherein each measurement includes detecting, using the detector, light of the light beam traveling from the light source along the beam path to the detector when the alidade is in a given rotational position about the first axis and the optical instrument is in a given rotational position about the second axis.