Stereo-SCIDAR: optical turbulence profiling with high sensitivity using a modified SCIDAR instrument

Abstract

The next generation of adaptive optics (AO) systems will require tomographic\nreconstruction techniques to map the optical refractive index fluctuations,\ngenerated by the atmospheric turbulence, along the line of sight to the\nastronomical target. These systems can be enhanced with data from an external\natmospheric profiler. This is important for Extremely Large Telescope scale\ntomography. Here we propose a new instrument which utilises the generalised\nSCIntillation Detection And Ranging (SCIDAR) technique to allow high\nsensitivity vertical profiles of the atmospheric optical turbulence and wind\nvelocity profile above astronomical observatories. The new approach, which we\nrefer to as 'Stereo-SCIDAR', uses a stereoscopic system with the scintillation\npattern from each star of a double-star target incident on a separate detector.\nSeparating the pupil images for each star has several advantages including:\nincreased magnitude difference tolerance for the target stars; negating the\nneed for re-calibration due to the normalisation errors usually associated with\nSCIDAR; an increase of at least a factor of two in the signal-to-noise ratio of\nthe cross-covariance function and hence the profile for equal magnitude target\nstars and up to a factor of 16 improvement for targets of 3 magnitudes\ndifference; and easier real-time reconstruction of the wind-velocity profile.\nTheoretical response functions are calculated for the instrument, and the\nperformance is investigated using a Monte-Carlo simulation. The technique is\ndemonstrated using data recorded at the 2.5 m Nordic Optical Telescope and the\n1.0 m Jacobus Kapteyn Telescope, both on La Palma.\n