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The prime focus echelle spectrograph and the MSS

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 ($\rm R\sim20000$) directly at the prime focus. Refer to the first group of such tasks, for instance, the observations of absorption spectra of quasars with $\rm V\gt 15-16$, 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 ($P\sim 1^h$) 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 $\rm tg \Theta_b$ and the collimated beam diameter $\rm d_{coll}$. The diffraction grating employed in quasar observations with the MSS has $\rm tg\Theta_b=0.47$ with a collimated beam of d_coll=258mm. Hence we obtain that the use of the classical echelle with $\Theta_b=63.5\degr$ 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.