In adaptive optics

Adaptive optics

Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

, the Greenwood frequency is the frequency or bandwidth

Bandwidth

Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context...

 required for optimal correction with an adaptive optics system. It depends on the transverse

Transverse

Transverse may refer to:*Transversality, a concept related to the intersection of manifolds in topology*Transverse City, an album by Warren Zevon...

 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

Wavefront

In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...

 needs to be corrected is higher, and vice-versa.

The Greenwood frequency is given by



With the zenith

Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

angle, the windspeed as function of height and the so-called atmospheric turbulence constant structure function, a measure of the turbulence strength as function of height.