  A sample of field early-type galaxies (E/S0) at intermediate redshift
($z\sim0.1-0.6$) is selected, based on morphology and colours from HST-WFPC2
parallel images. Photometric structural parameters (effective radius
$R_{\tx{e}}$ and effective surface brightness $SB_{\tx{e}}$) are derived
through the F606W and F814W filters, using luminosity profile fitting and
two-dimensional fitting techniques. The combined parameter that enters the
Fundamental Plane ($\log R_{\tx{e}}-\beta SB_{\tx{e}}$, with
$\beta\approx0.32$) is shown to suffer from significantly smaller uncertainties
(r.m.s. 0.03) than the individual structural parameters (e.g. $\sim 15$ per
cent r.m.s. on the effective radius).
  High signal-to-noise intermediate resolution spectra, taken at the ESO-3.6m,
yield redshifts for 35 galaxies and central velocity dispersions for 22
galaxies. Central velocity dispersions are derived using a library of stellar
templates covering a wide range of spectral types, in order to study the
effects of templates mismatches. The average random error on the central
velocity dispersion is found to be 8 per cent and the average systematic error
due to template mismatch is found to be 5 per cent. The errors on velocity
dispersion measurement and the effects of template mismatches are studied by
means of extensive Montecarlo simulations. In addition, we investigate whether
the determination of the velocity dispersion is sensitive to the spectral range
used, finding that the value of velocity dispersion is unchanged when the
spectral regions that include the absorption features Ca HK and NaD are masked
out during the fit.
