Transcript Document
Surface Layer SLODAR J. Osborn, R. Wilson and T. Butterley A prototype of a new SLODAR instrument has been developed at Durham CfAI and tested at the Paranal observatory. The instrument targets very wide double stars with separations of several arc-minutes to achieve profiling of the surface layer of turbulence with very high resolution in altitude (10m or less). From Telescope SLODAR (SLOpe Detection and Ranging) is an instrument which has been developed to profile the vertical distribution of optical turbulence, Cn2 (h). SLODAR measures the wavefront gradient as a function of position at the telescope pupil and then using triangulation can estimate the strength, altitude and velocity of each turbulent layer up to a maximum altitude. The vertical resolution of the system is set by the separation of the two target stars. Theoretically this separation could be arbitrarily large, however realistically it is limited by the field of view of the optical system and the physical size of the imaging camera. To avoid this limitation the system has been modified. A reflective wedge is used to split the two images of the stars onto separate cameras. Using this system high resolution profiles of the surface turbulent layer can be obtained. To Camera To Camera h D SLODAR geometry SL-SLODAR uses a reflective wedge to separate the images of the two stars in order to increase the field of view of the system Instrument • 14 inch f/11 Celestron • Reflective wedge to split the images of the two stars • 8 x 8 Shack Hartmann wavefront sensors • 2 x Andor Luca EMCCD • Frame rate = 40 Hz • Exposure time = 2 to 5 ms Data Reduction • Spot centroids are recorded and filtered to remove wind shake • Centroid cross covariance are calculated SL-SLODAR on the 14 inch Celestron at Paranal (left) next to the robotic SLODAR system (right) and auto covariance functions • Covariance functions are fitted to theoretical response functions to recover turbulence profile Example Profiles With a target separation of 16 arc minutes a resolution of 10 m is achieved. The histogram on the right is an example profile for 5 minutes of observation on the 7th June 2008 at Paranal. The negative values are partly due to noise and partly due to fitting errors with the theoretical response function. Example profile from SL-SLODAR. 2D centroid cross covariance plot (left), radial centroid covariance along direction of star separation (solid line, central plot) and transverse direction for comparison (dashed line, central plot) and the resulting Cn2 profile with 10 m resolution. r0 = 10 cm. Temporal De-correlation The plots below show the 2D centroid cross covariance with an increasing temporal delay between the two stars. The peak corresponds to a turbulent layer just above the ground. The peak is seen to be moving, the direction and speed in which the correlation peak moves shows the velocity of the turbulent layer. The well defined motion verifies that the detected layer is associated with the surface layer wind (rather than local turbulence within the telescope tube or instrument). Example Cn2 profile, r0 = 25 cm Contact: [email protected] t=0 t = 5 ms References: • Wilson (2002), MNRAS 337, 103. • Butterley et al. (2006), MNRAS 369, 835. t = 10 ms t = 15 ms t = 20 ms