In the present version we will be concerned with the standard
configuration of CASPEC, the Cassegrain Echelle Spectrograph which is
in operation at the 
Cassegrain focus of the ESO 3.6 m telescope.
The instrument has been described in detail by D'Odorico and Tanné
(1984) we will include here a summary of the main instrumental characteristics
which are relevant for the analysis of the data.
In its standard configuration CASPEC uses a
a 31.6 lines/mm echelle grating together with a 300 lines/mm grating
cross disperser. A short focal length camera (f/1.46) focuses the beam into
a thinned, back illuminated CCD consisting of 
pixels. One pixel on the detector corresponds to an entrance aperture
or 
seconds of arc on the sky, the first dimension being in the
dispersion direction. The table 
shows the change in resolution
with the order number.
In this configuration, the spectrograph records in a single CCD frame
an 
Å wide portion of the spectrum of objects with 
with a resolving power of 
and a signal-to-noise ratio
. This magnitude limit is set primarily by the
readout noise of the chip (50 electrons rms) and by the maximum exposure
time of 
min before contamination by cosmic rays becomes a problem.
Fainter objects can be observe at lower resolution by binning the CCD data.
Scattered light.
Assuming a plane grating, used in near--Littrow mode, the blaze
function R at wavelength 
is approximated by
where 
is a grating `constant' with value between 0.5 and 1, and
, in which m is the order number, and
is the central wavelength of order m. Both parameters are
related through the grating `constant' k by 
.
In table we include approximate values for
the parameters k and 
. These are mean values, given that the actual
values for a given observation
are a function of the order number and depend also on the instrumental set up.
