  We propose a design of an adaptive optics (AO) system for the high-resolution
fiber-fed echelle spectrograph installed at the Nasmyth focus of the 6-m BTA
telescope at the Special Astrophysical Observatory (SAO) of the Russian Academy
of Sciences (RAS). The system will be based on a pyramid wavefront sensor and
benefit from the experience of the Laboratoire d'Astrophysique de Marseille
team in the field of adaptive optics. The AO will operate in the visible domain
of 430-680 nm, in an f\30 input beam and provide correction for the on-axis
source only. The main challenges in this particular design are insetting
inserting the AO into an existing optical system and maintaining the focal and
pupil planes configuration, fitting within the instrument's flux budget as well
as limitations on the total cost of the AO bench. According to the current
design, the AO bench will use an additional relay consisting of 2 spherical
mirrors to re-collimate the beam and project the pupil onto a small deformable
mirror. A dichroic splitter will be used to longwave component to the pyramid
wavefront sensor branch based on refractive optics only. Using off-the-shelf
components only we can reach the instrumental wavefront error of 0.016 waves
PTV with a 20 nm bandpass filter at 700 nm. Using folding mirrors and
refocusing of the fiber's microlens we restore the nominal geometry of the beam
feeding the spectrograph. The final goal for the AO system is to increase the
energy concentration in spot at the spectrograph's entrance, and our
preliminary modelling shows that we can gain by factor of 69.5 with the typical
atmospheric conditions at SAO RAS.
