Design and Improvements of the Control System for LAMOST Spectrographs

LAMOST is a special Schmidt telescope with 16 spectrographs. Through these spectrographs, it can detect 4000 stellar spectra via optical fibers. Before the year of 2017 LAMOST's spectrograph only work on low resolution spectrograph(LRS) mode, and recently we have finished the update of optical, mechanical structure and control system of these 16 spectrographs which could switch working mode between low resolution and middle resolution spectrograph(MRS) to meet the needs of LAMOST Phase II Sky Survey. Due to the strict optical performance requirements and the close arrangement of the optical equipment on the spectrograph platform, the control system must be quite accurate, stable and reliable. In this paper, we mainly describe the design and improvements of the spectrograph control system of the LAMOST's spectrographs, including shutter sub-system control, back-illuminate sub-system control, LRS/MRS switch sub-system control, camera lens electric focus sub-system control, and some other sub-systems in LAMOST's spectrograph control system. What's more, there are also some connections between different sub- systems. As a result, we use FPGA chip as the main spectrograph controller, and make some improvements not only on host-computer software program, but also on slave-FPGA controller software and hardware design. The FPGA controller does some logical judgements according to the feedback information provided by the position sensor and the working mode designed to suit for different working condition. In this way, we make the spectrograph work more accurate and stable, and make it more safety and reliable especially on switching between LRS mode and MRS mode. Through those design and improvements on spectrograph's control system mentioned in this paper, LAMOST could get more high-quality star spectral data from its 16 spectrographs.