Surface Layer SLODAR

1
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.
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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 and auto covariance
  functions are calculated
• Covariance functions are fitted to theoretical
  response functions to recover turbulence profile

SL-SLODAR on the 14 inch Celestron at Paranal
(left) next to the robotic SLODAR system (right)
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

• References
• Wilson (2002), MNRAS 337, 103.
• Butterley et al. (2006), MNRAS 369, 835.

Contact james.osborn_at_durham.ac.uk