Atmospheric turbulence can severely degrade the quality of astronomical images obtained by large ground based telescopes. The effects of atmospheric turbulence on imaging of the night sky, recognised by us as twinkling of the star light, effectively blurs the images formed by the telescopes. A standard technique employed to compensate for this is the use of Adaptive Optics (AO). Here, the optics of the telescope is designed to actively compensate for the effect that the turbulent layers in the atmosphere have on the light passing through them. As one might expect, the design and performance of these systems is integrally linked to the strength of the turbulence at a particular observing site pointing to the need for techniques to asses the strength and character of the turbulence . In addition the interpretation of corrected images requires a knowledge of the atmospheric turbulence profile in the viewing direction of the telescope. SCIDAR is a techniques which can address these needs.

SCIDAR (SCIntillation Detection And Ranging) is a remote-sensing technique that can be used to measure the strength of the turbulence and its dependence on altitude (other techniques also exist). The method is based on an analysis of stellar scintillation images produced by the turbulent layers. These stellar scintillation images can be likened to the flying shadow patterns viewable at the bottom of a swimming pool when light passes through the water. The effect is brought about in this instance due to ripples at the water air interface acting like lenses and focusing the light to different regions.

Stellar scintillation is due to spatial differences in refractive index in the atmosphere, an optical wave propagating through the atmosphere will experience fluctuations in the optical path length though different layers due to these. At the ground this gives rise to scintillation patterns which vary on a time scale of the order of thousands of a second. Scintillation is caused almost exclusively by small temperature variations (on the order of 0.1 - 1^oC) in the atmosphere, resulting in index-of-refraction fluctuations (these fluctuations are on the order of 1 part in a million). The undulation in the atmosphere acts as a lens, focusing the starlight just as the water ripples did in the case of the pool of water.

Binary star SCIDAR is an established technique which uses scintillation images produced by binary star targets to asses turbulence. This serves as a useful tool in characterizing the quality of astronomical sites. However, in terms of its use for minute to minute assessment of turbulence along the direction of viewing interest of an astronomical instrument it suffers from a significant shortfall in that it relies on the user being able to locate a suitable binary pair in the direction of interest on which to home in the SCIDAR instrument. At any one time binary star SCIDAR has a limited number of useful targets in the night sky with which to work. The potential for adaptive optics instruments to benefit from real-time SCIDAR data is pushing for a solution to this problem (AO and MCAO systems performance can be improved by knowing the isoplanatic angle and the height of the strongest turbulent layer).

At the applied optics group in Galway we are engaged in development and testing of a Single Star SCIDAR (SSS) system for quantifying the strength of atmospheric turbulence. Using single star sources offers the promise of obtaining larger sky coverage. The key benefit of the system is that, if successful, it should open up the possibility of real time monitoring of turbulence in the direction of viewing interest for scientific instruments. Using single star targets one is confronted with the challenge of retrieving height profiles of the turbulence without the benefits of the triangulation ranging associated with the binary star counterpart. The system uses large data sets of short exposure scintillation images for a single star source together with a generalised SCIDAR approach to locate and quantify the strength of atmospheric turbulence. This is used to retrieve C_n²(h) a key metric quantifying the strength of the turbulence at different heights in the atmosphere.

With the basics of the system in place we are now moving on to field testing of the SCIDAR system. Archived data from the system will aid in optimising the algorithm for inferring C_n²(h) profiles. Ultimately, with the optimised algorithm in place, a fully automated version of the system is possible with the promise of improved sky coverage for assessing atmospheric parameters needed by adaptive optics systems

Basics of SCIDAR (SCIntillation Detection And Ranging)

Feasibility of Single Star Scidar & Data Processing

C_n²(h) Inversion

Preliminary Results and Data Processing 

The System in More Detail

 Publications and Presentations

 Links to Useful SCIDAR Sites