Correction of photometric scintillation noise via tomographic wavefront sensing: simulation and on-sky demonstration

Atmospheric scintillation noise severely limits the precision of time-resolved photometry for ground-based observations of bright stars. We describe developments of a method to correct this noise, for large and extremely large telescopes, via tomographic wavefront sensing. Wavefront sensor data for multiple reference stars is used to produce a 3D model of the instantaneous aberrations induced by atmospheric turbulence above the telescope. If the altitudes and relative strengths of the turbulent layers are known, then the phase aberrations of the wavefront at each height can be determined using tomography. This 3D model can then be used to calculate the propagation of the wavefront to ground level, and hence to estimate and correct the intensity fluctuations due to scintillation for a given target in the field of view. Potentially, this technique can be applied to the wavefront sensors of existing tomographic AO systems, with the scintillation correction applied and optimised in post processing. The method has been tested extensively in simulations. For example, for tomography using the 4 laser guide star asterism of the VLT, our simulations suggest that the RMS photometric noise for bright stars (which will be limited by scintillation) could be reduced by a factor of four in typical conditions. The method has also been tested in an on-sky demonstration, using the Orion Trapezium asterism as the reference stars for tomographic wavefront sensing on the Isaac Newton Telescope in La Palma.