A Concept Design of the Positioner of the Optical Fibres of the LAMOST

Abstract

LAMOST is a large multi-object fibre spectroscopy telescope. With a 5 degree field of view (FOV) and 20m focal length, the 1.75m focal surface is so large that the spectra of up to 4000 galaxies could be simultaneously observed. However the design and manufacture of the positioning device of such a large number of optical fibres in such a large FOV will be challenging. Firstly, the differential atmospheric refraction become very restrictive for the positioning of the fibres. The difficulty for the positioning of the fibres also comes from the large number of fibres. In this paper we propose a design concept for the positioning system of optical fibres for the LAMOST. In our consideration we will emphasize the effect of differential refraction and blind area on the observation. Wallace and Tritton (1979), Cohen and Cromer (1988) and Donnelly and Brodie (1993) have shown that the effect of differential refraction for the large field telescope is important. My colleagues Sun and Hu (1996) calculated the effect of differential refraction on the LAMOST. The results show that the image motion due to differential refraction across the 5 degree field from -0.75 to +0.75 hours at the LAMOST site (φ= 400.4) is unacceptably large [Fig. 1]. The maximum relative displacement of the objects in the 5 degree FOV of LAMOST will be larger than 4 arcsec in some fields. Wallace and Tritton (1979), Cohen and Cromer(1988), and Donnelly and Brodie (1993) have shown the patterns