The Fried parameter or Fried's coherence length (commonly designated as
r
0
) 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. As such, imaging from telescopes with apertures much smaller than
r
0
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
r
0
(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
λ
can be expressed in terms of the so-called atmospheric turbulence strength
C
n
2
(which is actually a function of temperature fluctuations as well as turbulence) along
z
′
the path of the starlight :
r
0
=
[
0.423
k
2
∫
P
a
t
h
C
n
2
(
z
′
)
d
z
′
]
−
3
/
5
where
k
=
2
π
/
λ
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
ζ
, the line of sight passes through an air column which is
sec
ζ
times longer, producing a greater disturbance in the wavefront quality. This results in a smaller
r
0
, so that in terms of the vertical path z, the operative Fried parameter
r
0
is reduced according to:
r
0
=
[
0.423
k
2
sec
ζ
∫
V
e
r
t
i
c
a
l
C
n
2
(
z
)
d
z
]
−
3
/
5
=
(
cos
ζ
)
3
/
5
r
0
(
v
e
r
t
i
c
a
l
)
.
At locations selected for observatories, typical values for
r
0
range from 10 cm for average seeing to 20 cm under excellent seeing conditions. The angular resolution is then limited to about
λ
/
r
0
due to the effect of the atmosphere, whereas the resolution due to diffraction by a circular aperture of diameter
D
is generally given as
1.22
λ
/
D
. Since professional telescopes have diameters
D
≫
r
0
, they can only obtain an image resolution approaching their diffraction limits by employing adaptive optics.
Because
r
0
is a function of wavelength, varying as
λ
6
/
5
, its value is only meaningful in relation to a specified wavelength. When not stated explicitly, the wavelength is typically understood to be
λ
=
0.5
μ
m
.
