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For high spectral resolution observations the devices
permanently located at the N2 focus of the BTA are used (Klochkova, 1995,
Panchuk, 1995). However for a number of tasks to be performed, it is needed
to rule out the influence of the second and third mirrors, i.e. to realize
the high spectral resolution () directly at the prime focus.
Refer to the first group of such tasks, for instance, the observations of
absorption spectra of quasars with
, when the S/N ratio at an
hour exposure is rarely higher than S/N=5. The observations of brighter
objects at short (comparable with the signal read-out time) exposures, for
instance, the observations of supershort-period (
) cepheids of
SX Phenix type at different pulsation phases or the observations of fast
rotators with a spectrophotometrically inhomogeneous surface, refer to the
second group. To the third group belong the spectropolarimetric observations
for which a contribution of instrumental polarization on the third (flat)
mirror of the BTA is important. In all these cases the gain due to
the elimination of losses of light at the second and third mirrors and
instrumental polarization at the third mirror is of principle importance.
As compared to the Nasmyth-2 focus echelle spectrometers, the placement
of the echelle spectrometer at the prime focus ensures a 1.4-fold minimum
gain in light
by removing the
second and third mirrors from the optical path
(the reflection coefficient of a
fresh aluminium coating is taken equal to 0.85).
Examine the principal parameters of the PF echelle spectrometer, which is
equivalent in spectral resolution to the CCD-equipped MSS (Panchuk, 1995).
The spectral resolution is proportional to the product of the tangent of the
diffraction grating blaze angle and the collimated beam
diameter
. The diffraction grating employed in quasar observations
with the MSS has
with a collimated beam of
dcoll=258mm. Hence we obtain that the use of the classical echelle with
reduces by nearly a factor of 5 the diameter of the
collimated beam of the echelle spectrograph of the same resolution. Such a
spectrograph will have the overall dimensions allowing it to be operated in
the prime focus cage. Therefore, if the throughout of this echelle
spectrometer at the prime focus compares favourably with that of the MSS at
the N2, the manufacture of such an apparatus is worthwhile.