Shifting Focal Point of Light Source to Decrease FRD of Astronomical Optical Fibers in LAMOST

The light from out space is very weak and to guide the light into the optical fiber efficiently in astronomical spectrum telescope is of great importance for spectrum analysis. And the FRD of optical fiber can also decrease the utilization of the light of celestial body for the expending diameter of the out spot from the optical fiber, which makes the spot more diffuser and decrease the distribution of energy density. The light from out space is focused to the focal plane in the telescope to couple into the fiber, but the focal point of the star light can be hardly controlled to be fixed on the incident end of the fiber, a common method of spinning the fiber to aim at the focal point automatically can efficiently track the path of the light point but not the focal point. So to optimize the transmission of light in the fiber can begin with FRD control by shifting the focal point of light source. Experiments results of testing the FRD of fibers in LAMOST with two kinds of light sources and two kinds of CCDs shows that shifting the focal point back behind the incident end of optical fiber can decrease FRD and the out focal ratio is more stable, the coefficient index can be up to 0.999.