The stray light uniformity is one of the important factors affecting the signal-to-noise ratio of the optical astronomical telescope. It will cause regional differences in the background intensity of the detector image, resulting in a decrease of the differential photometry accuracy. The source that affects stray light uniformity is the inconsistency of the brightness of the sky background, which comes from moonlight, bright star, and city lighting pollution. During CCD reduction, the effect of background uniformity cannot be eliminated by dividing the flat field.
Star deletion method is used in real-time stray light analysis. It’s very convenient to achieve a ‘clear’ background image without stars in MATLAB. A contour map of stray light distribution for each object image will be given to demonstrate the background uniformity directly. The stray light uniformity analysis method is implemented by the following steps: 1) CCD reduction, including preprocessing of an object image with bias and flat field; 2) Histogram generation, performing star subtraction automatically based on ADU value and frequency; 3) Background stray light contour map generation, stray light uniformity and other parameters calculations. This method will calculate the uniformity of image surface in real time, provide background intensity distribution, statistical data of the CCD image and suggestion on compare star selection during CCD data processing and improve the photometry accuracy.
Aerodynamic analysis is a crucial part of evaluating dome seeing, which is one of the main factors affecting telescope image quality. Due to the large volume and high heat quantity inside the dome, dome seeing is a common issue. To characterize the thermodynamic performance of the 2.16-m telescope at the Xinglong Observatory, we describe computational fluid dynamic analyses for modeling the effects of passive ventilation as part of a preliminary study for a dome venting system. The aerodynamic modeling is built based on the structures of telescope and enclosure. In addition, the distribution of the temperature and the airflow around the enclosure are presented in several simulations with different slit orientations, including various wind–telescope relative azimuth angles. The airflow distribution was studied for two cases. The temperature and turbulent contour maps show that the current passive ventilation can cause turbulence and influence the accuracy of the image. The dome seeing is estimated using a postprocessing analysis based on the mechanical turbulence and temperature variations along the optical path. The results of dome seeing gave a suggestion of venting strategy.
In optical astronomical telescopes, the primary baffle is a tube-like structure centering in the hole of the primary mirror and the vanes usually locate inside the baffle, improving the suppression of stray light. They are the most common methods of stray light control. To characterize the performance of primary baffle and vanes, an empirical comparison based on astronomical observations has been made with Xinglong 50cm telescope. Considering the convenience of switching, an independent vanes structure is designed, which can also improve the process of the primary mirror cooling and the air circulation. The comparison of two cases: (1) primary baffle plus vanes and (2) vanes alone involves in-dome and on-sky observations. Both the single star and the various off-axis angles of the stray light source observations are presented. The photometrical images are recorded by CCD to analyze the magnitude and the photometric error. The stray light uniformity of the image background derives from the reduction image which utilizes the MATLAB software to remove the stars. The in-dome experiments results reveal the effectiveness of primary baffle and the independent vanes structure. Meanwhile, the on-sky photometric data indicate there are little differences between them. The stray light uniformity has no difference when the angle between the star and the moon is greater than 20 degrees.