The purpose of the analysis was to discriminate statistically
between the different contributions to the overall image spread
and quantify in particular the local seeing effects.
The main analysis guideline will be given by the assumption that
the overall image quality can be parameterized as a sum of various
terms dependent on few driving quantities such as the
zenithal distance 
and various temperature deltas. Recalling that the amplitudes
of various seeing FWHM contributions sum up with the power 5/3, while all
other contributions should be added quadratically, one has:
where
is the telescope/instrument image FWHM. For the HRCam
case this was reported after calibration tests to be about 0.35 arcsec,
practically independent on the zenithal angle [Racine 92].
is the natural seeing,
here defined as the contribution
which is independent from particular conditions of telescope and dome.
From equation (
) one has
where 
is the natural seeing at zenith.
is the local seeing, for which parameterizations
will be sought with respect to various temperature deltas
as well as wind speed and direction.
is the contribution due to wind shaking of the
telescope.
) one
will assume that local contributions to the seeing angle
depend on the power 6/5 of the corresponding 
s:
The following thermal indices will be considered
(see fig. and table ):
Since the scattering of the global seeing is exceedingly dominated by the fluctuations of natural seeing, it is not advisable to use direct multivariate least square regressions to evaluate all contributions in one computation and a more careful stepwise approach is required. Therefore we shall attempt to isolate the different influences and derive their parameterizations by using suitable subsets of the data.
