Sodium layer monitoring at Calar Alto by LIDAR

Observations have shown the presence of sodium layer centroid height variations of a few hundred metres on timescales of tens of seconds. As quality laser guide star (LGS) plus adaptive optics (AO) assisted astronomy, especially on large (8m+) telescopes, will require optimal scheduling of observations and regular laser and wavefront sensor focussing at sites where sporadic sodium layers are frequent, an `easy to use' sodium layer monitor is required. LIDAR offers a convenient means to achieve this. By pulsing the outgoing sodium laser and performing time-of-flight measurements on the returned photons we can acquire the altitude profile of the sodium layer. Unfortunately, conventional LIDAR requires the laser duty cycle to be very low, therefore large integration times are required. However, by using a cross-correlation technique the duty cycle can be increased to 50%, which gives far better performance. We present the details of this technique which involved amplitude modulation of the MPIA/MPE ALFA cw laser, as well as the following results of such LIDAR measurements performed in October 1999 at the 3.5m telescope at Calar Alto Observatory in Spain. The altitude of the sodium layer at Calar Alto on 17^th and 18^th October 1999 was found to be at 90±3km and there is evidence for sporadics on one of two nights with sporadic layer FWHM varying from approximately 240 to 350m. In addition, a noticeable layer FWHM change (excluding the sporadic layer) from approximately 13 to approximately 5-7km was observed over the two nights. After flux and altitude calibration and correction of the projected altitude range, a very good agreement is found between sodium layer profiles derived from an auxiliary telescope and 3.5m telescope LIDAR observations. Using an intensity weighted centroid algorithm the centroid height of the sodium layer was observed to have a variation of <500m in approximately 10 minutes. Although, shorter timescale variations may be have been present, poor observing conditions and resulting reduced S/N prevents this analysis.