The Error Analysis and Correction of NVST's Long-Term Tracking

Though NVST is equipped with an optical auto-guiding system, the tracking accuracy is still affected by the mechanical structure of telescope, guiding system and derotator platform. In order to correct the long-term tracking error caused by above factors and improve the tracking accuracy, the solar images on the F₃ focal plane are used. Images are cross-correlated in real time with a previously recorded reference image of the same observation target. The drive signal for the image motion is generated by long-term average of the positions of the maximum of the cross correlation. The experiment in TiO channel of high-resolution imaging system in NVST shows the tracking error by feature tracking in four hours is 0.52"(RMS). Thus this result can demonstrate that for NVST, long-term tracking accuracy can be improved by using the image motion on coude focal plane in closed-loop telescope tracking.