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 reconstruction techniques to map the optical refractive index fluctuations, generated by the atmospheric turbulence, along the line of sight to the astronomical target. These systems can be enhanced with data from an external atmospheric profiler. This is important for Extremely Large Telescope scale tomography. Here we propose a new instrument which utilises the generalised SCIntillation Detection And Ranging (SCI-DAR) technique to allow high sensitivity vertical profiles of the atmospheric optical turbulence and wind velocity profile above astronomical observatories. The new approach, which we refer to as 'Stereo-SCIDAR', uses a stereoscopic system with the scintillation pattern from each star of a double-star target incident on a separate detector. Separating the pupil images for each star has several advantages including: increased magnitude difference tolerance for the target stars; negating the need for re-calibration due to the normalisation errors usually associated with SCIDAR; an increase of at least a factor of two in the signal-to-noise ratio of the cross-covariance function and hence the profile for equal magnitude target stars and up to a factor of 16 improvement for targets of 3 magnitudes difference; and easier real-time reconstruction of the wind-velocity profile. Theoretical response functions are calculated for the instrument, and the performance is investigated using a Monte-Carlo simulation. The technique is demonstrated using data recorded at the 2.5 m Nordic Optical Telescope and the 1.0 m Jacobus Kapteyn Telescope, both on La Palma.

Atmospheric scintillation in astronomical photometry

“…We have seen periods of strong, high layers which survive for several hours and we have also seen small bursts of activity lasting only a few minutes (Avila, Vernin & Cuevas 1998;Shepherd et al 2013). The altitude and strength of these layers varies with time, having significant impact on the scintillation noise during observations.…”

“…It is therefore possible to estimate the scintillation noise using Here we use stereo-SCIDAR Shepherd et al 2013), a new high-resolution SCIDAR instrument, which can automatically obtain concurrent turbulence velocity and turbulence strength profiles. Stereo-SCIDAR was installed on the 2.5 m Isaac Newton Telescope (INT), La Palma, for three two-week campaigns in 2014 and on the 1 m Jacobus Kapteyn Telescope (JKT) for three two-week campaigns in 2013.…”

Modelling and prediction of non-stationary optical turbulence behaviour

“…The starting point to estimate d is the average PSD -as shown in Figure 6 -together with the expression for the model PSD (5). By taking the log of the periodogram, the fractional power d can be estimated by a straightforward linear regression approach.…”

Scintillation correction for astronomical photometry on large and extremely large telescopes with tomographic atmospheric reconstruction

“…Here we take the shot noise to be the fundamental limit to the precision and assume other noise sources to be negligible. Using recent results from a new scintillation detection and ranging (SCIDAR) atmospheric optical turbulence profiling instrument, Stereo-SCIDAR Shepherd et al 2013), over 30 nights distributed between 2013 May and September, on La Palma, we can calculate the median atmospheric turbulence profile and then use this to estimate the effective strength at a single turbulent layer at 10 km in order to conserve the median scintillation index. This is done to simplify the calculations and simulations for the rest of this work.…”