The Fried parameter or Fried's coherence length (commonly designated as ) is a measure of the quality of optical transmission through the atmosphere due to random inhomogeneities in the atmosphere's refractive index. In practice, such inhomogeneities are primarily due to tiny variations in temperature (and thus density) on smaller spatial scales resulting from random turbulent mixing of larger temperature variations on larger spatial scales as first described by Kolmogorov. The Fried parameter has units of length and is typically expressed in centimeters. It is defined as the diameter of a circular area over which the rms wavefront aberration due to passage through the atmosphere is equal to 1 radian. For a telescope with an aperture, , the smallest spot that can be observed is given by the telescopes Point spread function (PSF). Atmospheric turbulence increases the diameter of the smallest spot by a factor approximately (for long exposures). As such, imaging from telescopes with apertures much smaller than is less affected by atmospheric seeing than diffraction due to the telescope's small aperture. However, the imaging resolution of telescopes with apertures much larger than (thus including all professional telescopes) will be limited by the turbulent atmosphere, preventing the instruments from approaching the diffraction limit.
Although not explicitly written in his article, the Fried parameter at wavelength can be expressed in terms of the so-called atmospheric turbulence strength (which is actually a function of temperature fluctuations as well as turbulence) along the path of the starlight :
where is the wavenumber. If not specified, a reference to the Fried parameter in astronomy is understood to refer to a path in the vertical direction. When observing at a zenith angle , the line of sight passes through an air column which is times longer, producing a greater disturbance in the wavefront quality. This results in a smaller , so that in terms of the vertical path z, the operative Fried parameter is reduced according to:
At locations selected for observatories, typical values for range from 10 cm for average seeing to 20 cm under excellent seeing conditions. The angular resolution is then limited to about due to the effect of the atmosphere, whereas the resolution due to diffraction by a circular aperture of diameter is generally given as . Since professional telescopes have diameters , they can only obtain an image resolution approaching their diffraction limits by employing adaptive optics.
Because is a function of wavelength, varying as , its value is only meaningful in relation to a specified wavelength. When not stated explicitly, the wavelength is typically understood to be .
- Fried, D. L. (October 1966). "Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposures". Journal of the Optical Society of America. 56 (10): 1372–1379. Bibcode:1966JOSA...56.1372F. doi:10.1364/JOSA.56.001372.
- For short exposures the observed spot will break up into a number of speckles. Each speckle will move around in time to integrate over a long exposure to a diameter approximately D/r0. The size of each speckle is given by the point spread function of the telescope.
- Hardy, John W. (1998). Adaptive optics for astronomical telescopes. Oxford University Press. p. 92. ISBN 0-19-509019-5.