Optical system, comprising - a signal input for receiving a signal beam, - a signal output for outputting the signal beam, - an auxiliary light source for emitting an auxiliary beam, the auxiliary beam being synchronous with the signal beam in angle and position, - a rotation system, the rotation system being rotatable about an axis of rotation, and - an axial feed-in mirror for feeding the signal beam and the auxiliary beam into the rotation system, - the rotation system having: ∘ a signal feed-out mirror for feeding the signal beam out of the rotation system, ∘ a reference mirror, and ∘ a position detector for detecting an auxiliary beam projection of the auxiliary beam, - the signal beam being deflectable from the signal input via the axial feed-in mirror to the signal feed-out mirror, and the auxiliary beam being deflectable via the axial feed-in mirror through the signal feed-out mirror onto the reference mirror, the auxiliary beam being reflectable back from the reference mirror at least in part to form a reflected auxiliary beam, - the optical system further comprising an angle sensor, by way of which an angular deviation between the auxiliary beam and the reflected auxiliary beam can be detected, - the axial feed-in mirror and the reference mirror being tiltable so as to orientate the signal beam parallel to the axis of rotation using the measured angular deviation, characterised in that - the auxiliary beam projection describes a second circular path when the auxiliary beam is orientated parallel to the axis of rotation and the rotation device is rotating, and - a reflected auxiliary beam projection on the angle sensor describes a first circular path when the rotation system is rotating, it being possible to influence a first radius (α r) of the first circular path by tilting the reflection mirror and first centre point coordinates (α x, α y) of the first circular path by tilting the axial feed-in mirror. - a signal input for receiving a signal beam, - a signal output for outputting the signal beam, - an auxiliary light source for emitting an auxiliary beam, the auxiliary beam being synchronous with the signal beam in angle and position, - a rotation system, the rotation system being rotatable about an axis of rotation, and - an axial feed-in mirror for feeding the signal beam and the auxiliary beam into the rotation system, - the rotation system having: ∘ a signal feed-out mirror for feeding the signal beam out of the rotation system, ∘ a reference mirror, and ∘ a position detector for detecting an auxiliary beam projection of the auxiliary beam, ∘ a signal feed-out mirror for feeding the signal beam out of the rotation system, ∘ a reference mirror, and ∘ a position detector for detecting an auxiliary beam projection of the auxiliary beam, - the signal beam being deflectable from the signal input via the axial feed-in mirror to the signal feed-out mirror, and the auxiliary beam being deflectable via the axial feed-in mirror through the signal feed-out mirror onto the reference mirror, the auxiliary beam being reflectable back from the reference mirror at least in part to form a reflected auxiliary beam, - the optical system further comprising an angle sensor, by way of which an angular deviation between the auxiliary beam and the reflected auxiliary beam can be detected, - the axial feed-in mirror and the reference mirror being tiltable so as to orientate the signal beam parallel to the axis of rotation using the measured angular deviation, characterised in that - the auxiliary beam projection describes a second circular path when the auxiliary beam is orientated parallel to the axis of rotation and the rotation device is rotating, and - a reflected auxiliary beam projection on the angle sensor describes a first circular path when the rotation system is rotating, it being possible to influence a first radius (α r) of the first circular path by tilting the reflection mirror and first centre point coordinates (α x, α y) of the first circular path by tilting the axial feed-in mirror.