dynamic telescope-to-fiber coupling correction

For smaller telescope diameters the seeing distortion is to a high degree only a movement of the star image in the focal plane, e.g. to >80% for the 35cm-diameter telescope we use here. Therefore it is assumed that, as a cheaper approximation to AO, a “tip-tilt correction” is sufficient.
The large optical bandwidth of the quad-PD in direct detection ensures a strong signal and fast reaction of the loop to the beam fluctuations. For larger telescopes, additionally their adaptive optics is necessary to be switched on, in order to couple into a single mode fiber efficiently. However, then still this stabilization scheme would be helpful to automatically lock to a AO-corrected star image. The coupling to the fiber is nevertheless variable. Therefore photometric signals for both telescopes are needed. Those are available from the autocorrelation parts of both correlator inputs. The cross-correlation signal is divided by both of them in order to obtain the true visibility value.