The OBS FSw command also asks for the duration of one phase (default 1.5 sec). Although existing equipment permits phase durations in the range from 10 msec to 2.5 sec, not all of this range is actually usable. While the long phase durations do not suppress efficiently atmospheric and receiver gain variations, phase durations shorter than 1 sec are not possible with the autocorrelator. If only the 100 kHz filter spectrometer is connected, phase durations as short as 50 msec have been used successfully. At still shorter phase durations data transfer errors set in. The limits used by OBS take these problems into account, and the use of the default value (1.5 sec) is recommended. In view of the receiver instabilities which are pronounced near 1 sec, it would be desirable, however, to use significantly shorter phase durations. An upgrade of the autocorrelator microprocessor hard and/or software would make this possible.