Statistics – Applications
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009otam.conf...23b&link_type=abstract
Optical Turbulence: Astronomy Meets Meteorology, Proceedings of the Astronomy Meets Meteorology, Proceedings of the Optical Turb
Statistics
Applications
Scientific paper
The scintillation of point-like objects is primarily caused by thermal fluctuations in the upper atmosphere. For it the scintillation index (σI2) is proportional to the height integral of Cn2(h) weighted by a height dependent function F(h) = hα where α = +5/3. For extended objects like the Sun or the Moon the height contribution to the (much smaller) scintillation is quite different. Because of their size the effects of the optical turbulence is averaged over an ever increasing area as the distance to the detector increases. Assuming vertical viewing, the area diameter increases like h*Ω where Ω is the angular diameter of the Sun or Moon. For Kolmogorov turbulence the function F(h) then still has the same shape, but with α = -1/3 so that the lower layers contribute more to scintillation. This makes it a good tool for the probing of the lower atmospheric layers. Unlike the σI2 for stellar scintillation, the σI2 for the Sun and the Moon is wavelength independent. Using an array of scintillometers one can probe the Cn2(h) distribution of those lower layers in a technique called SHABAR. SHABARs have been used in site testing for lower atmosphere probing for solar and nighttime telescopes. The aim is to establish the height to place telescopes, like the Advanced Technology Solar Telescope (ATST), to minimize boundary layer seeing effects. SHABAR site tests using the Moon are planned both for Arctic sites (Hickson's contribution to this meeting) and Antarctic sites (Storey's contribution to this meeting) where boundary layer heights are very site dependant reaching sometimes very small values. In my contribution I described some of the solar results related to the ATST site testing. The scintillation of planets have an F(h) function different in shape from that of the Sun or Moon. For low heights, where their beams still are narrow, F(h) has an α of +5/3 (as for stars); for large heights it is -1/3 (as for the Sun & Moon). For Mars the height contributions F(h) for seeing and scintillation are similar.
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