Laser guide stars as comparison stars: correcting scintillation noise

ABSTRACT The Earth’s atmosphere severely limits ground-based high precision photometry. Whilst adaptive optics can be used to improve image resolution, intensity fluctuations due to scintillation and atmospheric transparency variations remain. Scintillation noise cannot typically be corrected with a comparison star as it is produced by high altitude turbulence, and therefore the range of angles over which it is correlated is very small. Comparison stars can be used to correct for atmospheric transparency variations, however, its shot noise, as well as differences in the airmass along the lines of sight for each star, add noise to the calibration. These noise sources significantly limits ground-based observations of time-varying astronomical sources such as exoplanet transits. We propose a new technique to correct for these effects by superimposing a sodium laser guide star (LGS) with a science target star, therefore creating an artificial photometric reference beacon that passes along the same line of sight. The measured LGS photometry can then be used to correct the intensity variations of the target star due to scintillation. Simulation results exploring this proposed technique are presented along with results from an on-sky test of this experiment conducted in La Palma, Spain, using a simple instrument to image the LGS and the target star light sources separately onto a single detector. On-sky tests were able to reduce the variance of the light curve for the target star on average by a factor of $2.8\pm 0.6$. This demonstrates the technique and we expect that higher correction could be achieved.