A pyramid-based adaptive optics for the high-resolution echelle spectrograph at SAO RAS 6-m telescope

Abstract

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.