Greenwood frequency

In adaptive optics, the Greenwood frequency[1] is the frequency or bandwidth required for optimal correction with an adaptive optics system. It depends on the transverse windspeed and the turbulence strength in the atmosphere. This can be easily understood since if the turbulence moves over the telescope opening faster, the speed at which the wavefront needs to be corrected is higher, and vice-versa.

The Greenwood frequency is given by

f_{\mathrm G} = 2.31 \, \lambda^{-6/5} \left[ \sec \zeta \int_{\mathrm{Path}} C_n^2(z) \, v_{\mathrm{Wind}}(z)^{5/3} \, dz \right]^{3/5} [2]

With \zeta the zenith angle, v_{\mathrm{Wind}}(z) the windspeed as function of height and C_n^2(z) the so-called atmospheric turbulence constant structure function, a measure of the turbulence strength as function of height.

See also

References

  1. ^ Greenwood, Darryl P. (March 1977). "Bandwidth specification for adaptive optics systems". Journal of the Optical Society of America 67 (3): 390–303. Bibcode 1977JOSA...67..390G. doi:10.1364/JOSA.67.000390. http://www.astro.uu.nl/~werkhvn/study/Y5_07_08/master/papers/1977JOSA...67..390G.pdf. 
  2. ^ Tyson, Robert K.; Frazier, Benjamin W. (2004). Field guide to adaptive optics. SPIE Press. p. 14. ISBN 0819453196. http://www.slac.stanford.edu/spires/find/books/www?key=343315.