Similar statistical properties may be applied to
the index of refraction and one may define a
structure coefficient of the index of refraction
,
related to
by:
where is the wavelength.
Normally one considers as a reference the wavelength
= 500 nm and the previous equation becomes:
The seeing effect through an atmospheric layer of height
H can then be expressed an integral function of the index
of refraction structure coefficient ,
whereby the Fried parameter
is given by
where is the zenithal angle of the direction of
observation
.
Recalling equation (), the FWHM of the seeing disk
in arcsec is then given by:
where is the zenithal angle of the direction of observation.
For a vertical direction and = 500 nm, the
FWHM is expressed as:
Combining with equation (), for typical conditions of
astronomical mountain sites
(pressure 770 mb, temperature 10
) one obtains:
The diagram at fig. illustrates the order
of magnitude of the seeing effect with respect to a mean value of
and the integration distance.
Depending on the geometric scale of
the phenomenon causing seeing, the critical values of
will
be very different: for instance, if we set at
0.1 arcsec an
arbitrary threshold for "bad" seeing from a single cause,
the corresponding critical (mean) value of
will be