We have developed an improvement of the well-known SLODAR (Slope Detection and Ranging) technique which
allows us to monitor the ground layer throughout the night with resolution of a few tens of meters. We will present data
we have obtained during two one-week observing sessions at the Las Campanas Observatory, Chile. The first observing
session we have often observed a persistent, low ground layer which dominates the seeing. The second observing session
the free-atmosphere dominated the seeing. We will present the instrument design; real-time software; describe the
observing sessions, the data and results.
PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed Australian/European optical/infrared
telescope for Dome C on the Antarctic Plateau, with target first light in 2012. The proposed telescope is 2.4m diameter,
with overall focal ratio f/10, and a 1 degree field-of-view. In median seeing conditions, it delivers 0.3" FWHM wide-field
image quality, from 0.7-2.5 microns. In the best quartile of conditions, it delivers diffraction-limited imaging down
to 1 micron, or even less with lucky imaging. The areas where PILOT offers the greatest advantages are (a) very high
resolution optical imaging, (b) high resolution wide-field optical imaging, and (c) all wide-field thermal infrared
imaging. The proposed first generation instrumentation consists of (a) a fast, low-noise camera for diffraction-limited
optical lucky imaging; (b) a gigapixel optical camera for
seeing-limited imaging over a 1 degree field; (c) a 4K x 4K
near-infrared (1-5 micron) camera with both wide-field and diffraction-limited modes; and (d) a double-beamed midinfrared
(7-40 micron) camera.
In preparation to characterize the Giant Magellan Telescope site and guide the development of its adaptive
optics system, two campaigns to systematically compare the turbulence profiles obtained independently with
three different instruments were conducted at Las Campanas Observatory in September, 2007 and January 2008.
Slope detection and ranging (SLODAR) was used on the 2.5-m duPont telescope. SLODAR measures the C2n
profile as a function of altitude through observations of double stars. The separation of the observed double
star sets the maximum altitude and height resolution. Ground layer (altitudes < 1 km) and free atmosphere
turbulence profiles are compared with those obtained with a lunar scintillometer (LuSci) and a multi-aperture
scintillation sensor (MASS), respectively. In addition, the total atmospheric seeing was measured by both
SLODAR and a differential image motion monitor (DIMM).
We have investigated seasonal turbulence variation with altitude above the Siding Spring Observatory using the 24" telescope facility and interchangeable SCIDAR (Scintillation Detection and Ranging) and SLODAR (Slope Detection and Ranging) techniques. The latter proposed by Wilson triangulates the correlations through image motion of and between images of binary stars when imaged through multiple sub-pupils in a Shack-Hartmann configuration, so to determine the C2N(h) profile as a function of height above the telescope, and temporal evolution from subsequent short exposure, fast frame rate images. The simplicity of SLODAR pertains to the lack of restriction in coherence or fringe visibility (scintillation index) that underlies the SCIDAR technique, and that the available light is divided into N sub-pupils to be condensed to speckle star images, rather than spread across the full imaging array. However, with the like of f18 telescope being reduced in each sub-pupil to f60, using a 1-4 ms exposure intensified imager, we are still limited to magnitude 5-7 star pairs, of which there are precious few, and their associated angular separations limit the altitude resolution of the technique accordingly. To overcome some of these restrictions we have obtained data, and propose methods to process such, using the Galilean satellites of Jupiter (each mag 5-6), which change separation over the observing cluster of nights, and hence offer a changeable altitude sampling. These are unresolvable in the sub-pupil imaging system, but suffer less from the speckling effects of a scintillated point source. We present results to reinforce these premises.