However, an automatic procedure would have been much more efficient to obtain the best optimization. The correction signal is sent to the PZT actuator. When the PZT elongation exceeds a threshold value, the translation stage is used for desaturation. The fringe detection process works continuously as the OPD is linearly scanned by the translation stage. The integration time is set according to the coherence length of the fringe pattern and the scan speed.
Typical values are and a few , respectively, allowing an integration time of a few seconds.
If the visibility estimate increases above the alert threshold during the OPD scan, then the scan is stopped. A new visibility estimate, with a longer integration time, is performed. Comparison with another threshold, set for these new conditions, allows the system to decide whether fringes are actually there or not. In case of fringe detection, the servo tracking is automatically switched on, otherwise the scan resumes.