High precision guide method for the solar telescope

Abstract

A high-precision real-time guide method for solar telescope is introduced in this paper. Since the Solar Magnetism and Activity Telescope (SMAT) has operated for nearly 10 years at Huairou Solar Observing Station, National Astronomical Observatories, some devices have become aging and the system performance has greatly decreased. For performing the technical renovation and transformation, studying a new high-precision auto-guide system for solar telescope was an important and essential job instead of the old guide system. The auto-guiding system goes through a point-line-plane model development in solar telescopes. The new system was based on full-disk solar images which were collected from the main telescope in real-time. The technology of large area CCD array has already substituted the point or line array model in the auto-guide system. We locate the center of the full-disk solar images which recorded by large array CCD image sensor in real time and eliminate noise caused by bad weather, such as clouds and fog, as well as improving computational accuracy of centroid, by setting gray threshold, determining the location of centroid, and using the ratio of the length major axis to the length of minor axis to quantify the deviation from a perfect disk. The deviation is analyzed and changed into control signal, which, through connection-oriented TCP/IP network, drives the motor to adjust the axis of telescope. The system is close-loop control. The above-mentioned method, combining hardware architecture with software algorithms, improves the precision of guide along with stability. Moreover, the connection-oriented TCP/IP network improves the efficiency, reliability and compatibility of the system. An Ethernet interface is also provided for remote control. From the testing results, there was a periodic oscillation which badly influences the tracking accuracy of solar telescope in perpetual motion in RA direction. However, there was a stationary phase on which the deviation is within 3 pixels between two periodic oscillations last for 30 s. We could make a prediction to remind the observers when the telescope arrives the stationary phases during the solar magnetic field observing. In the observation, the precision of this new method is about 5 pixels up and down in the RA direction besides a few of points. In the DEC direction, the precision attained to 2.5 pixels. In addition, the position of the solar in the image was still stay where it was after one hour in the observation. The results show that the new method could make the solar telescope steadily accurately track the solar.