Adaptive optics with a Laser For Astronomy

TMR network: experiments using ALFA

The TMR Network has now completed its 4 years. Below are a few of the results that were obtained during that time.

Sodium Layer

It is well known that the sodium column density has diurnal and seasonal variations. Work carried out with the National University of Ireland, Galway has shown that in addition to this the vertical profile of the sodium layer can change significantly on timescales of less than 1 hour. This can be seen by measuring the intensity along the ALFA's laser guide star when viewed off-axis. Such variations have important implications for variability in both the brightness and focus of the laser guide star, which have direct consequences for the adaptive optics performance. In order to obtain some handle on the statistics of these changes, and how seriously they may compromise AO observations, a continuing program of Na layer monitoring is underway. Current results are given in a paper by O'Sullivan et al: ``The Mesospheric Sodium Layer at Calar Alto, Spain'' (49K gzipped postscript).

If the absolute Na column density can also be measured (eg by absorption of stellar light), it will also be possible to compare the predicted LGS brightness with that measured. This is beneficial to our understanding of the physics behind propogation through the atmosphere, scattering, and sodium excitation. There is a paper describing this by Ageorges et al: ``Atmospheric Sodium Column Density Monitoring'' (550K gzipped postscript).

A better method to make measurements of the sodium layer is to use the LIDAR technique. We now have our first results from such an experiment.

Scattered Light

At a busy observatory site, a bright laser can often be a nuisance to other observers. At worst it can completely ruin careful observations, and already at Calar Alto this has happened once. So that the necessary precautions can be taken to avoid future such `collisions', we must know the extent of the scattered light from the plume. Measurements of this have begun and are presented by Delplancke et al: ``LGS light pollution investigation at Calar Alto'' (455K gzipped postscript). A further benefit of this work is that by quantifying the scattered light, we know the power reaching the sodium layer which again helps to make predictinos about the brightness of the LGS and the AO performance.

Atmospheric Turbulence

Adaptive Optics performance is extremely dependent on the prevailing atmospheric conditions, and knowledge of the height and speed of the dominant turbulent layers allows one to estimate the quality of the correction and decide which observations are most suitable at that time. Work carried out by Imperial College, London UK with SCIDAR at Calar Alto has measured the Cn2 profile (atmospheric turbulence) and the velocities of the main layers. These are presented in a paper by Klückers et al: ``Profiling of atmospheric turbulence strength and velocity using a generalised SCIDAR technique'' (124K gzipped postscript). The results from this one night seem rather optimistic, and further observations have been carried out.

A software package which can model laser guide star and adaptive optics systems is being written by several TMR partners, and ALFA will be the first system it will attempt to model. It will allow us to peform a number of experiments, for example we can measure the Cn2 profile and velocities with SCIDAR, and model the wavefront distortions they produce using the software. These can then be compared to simultaneous measurements of the wavefront using ALFA - results will be available soon.

Tip-Tilt from a LGS

Determining the tip-tilt from a LGS is a major concern. Working with the Osservatorio Astronomico di Acetri, Italy, we have attempted to measure this for the first time using the perspective method during 1998. We succeeded in demonstrating that it is in principle possible; however technical difficulties (finding and tracking on the LGS from a second telescope is far from easy!) mean that a second attempt will be necessary to finally clinch it.

partners in the TMR network

``Laser Guide Stars for 8-metre class Telescopes''

CRAL - Observatoire de Grenoble
European Southern Observatory
Imperial College
Instituto di Astrofisica de Canarias
Max-Planck-Institut für extraterrestrische Physik
Osservatorio Astronomico di Arcetri
National University of Ireland, Galway