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Optical design for Antarctic Bright Star Survey Telescope



Testing BSST on the roof of Nanjing Institute of Astronomical Optics and Technology mirror lab & BSST Images with 7' FOV (field of view).

Dome A, the summit of Antarctic Plateau, has been supposed to be one of the most inaccessible parts of the continent where the elevation height is 4091 m, and the average temperature of the year is about -56 ℃. In January 2005, the Chinese National Antarctic Research Expeditions arrived at Dome A for the first time, and set up the Kunlun Station on January 27th, 2009, which makes the Dome A region become one of the best candidates for astronomical observations. Site testing over the past decade reveals that atmospheric scintillation at Antarctica Plateau is weaker than that at temperate observatories, and the free atmosphere seeing is better, transparency is greater and turbulent boundary layer is lower. Moreover, the in-situ Antarctic telescopes manage to continuously observe for several months in polar nights, which is quite suitable for time domain astronomy. Several Chinese Antarctic telescopes, including Chinese Small Telescope Array and two Antarctic Survey Telescopes, have been deployed on Dome A by Chinese Center of Antarctic Astronomy.

The Antarctic Bright Star Survey Telescope (BSST), for exoplanets survey in a relatively large sky area around the South Celestial Pole, has been successfully developed by Nanjing Institute of Astronomical Optics and Technology (NIAOT), Chinese Academy of Sciences, and University of Science and Technology of China. The results are reported in Chinese Optics Letters, Vol. 13, No. 11, 2015 (Z.Y. Li et al., Optical design for Antarctic Bright Star Survey Telescope).

BSST has an aperture of 300 mm, a field of view of 4.8°, and 7 filters, which meet wavelength coverage from 0.36 μm to 1.01 μm. The telescope requires qualified operation in domestic sites and Antarctic site, working within the temperature range from -80 ℃ to 40 ℃. The difficulties are that mirrors will frost when the ambient atmosphere warms up quickly in Antarctica, and the temperature deformation, especially for the optics, will possibly degrade the optical performances.

In the research, optical design of the BSST, tolerance analysis and alignment plan have been conducted. Based on the fully understanding of Antarctic telescopes construction, Dr. Zhengyang Li, from this research group, says that the optical design and tolerance analysis guarantee the quality of BSST. The test observation in Yunnan Observatory reveals that a single star image with 120 seconds exposure is deep to B=16.7, V=17.1 and R=17.1 magnitude in clear nights, and offered a photometric precision of 4.5 millimagnitude for a 12 magnitude stars in open band. Transiting events of exoplanets HAT-P-3b and HAT-P-12b was successfully observed.

The telescope is planned to be installed at Antarctic Zhongshan Station in January 2016, and operates for 2 or 3 years before carrying out transits survey at Dome A. BSST, a unique and powerful telescope, convinces the astronomers to make significant discoveries in Antarctica.



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在“人类不可接近之极”建造南极亮星巡天望远镜



SST在南京天文光学技术研究所镜面实验室楼顶测试和BSST拍摄的7角分视场星图

南极大陆的最高点冰穹A,海拔4091 m,年平均气温-56℃,曾经被认为是人类不可接近之极。2005年1月,中国南极科学考察队首次登顶,2009年1月27日建立中国南极昆仑站。从此,具有优良台址条件的冰穹A地区作为台址候选成为可能。十多年的台址监测结果显示,与常规中纬度台址相比,南极高原天文观测台址具有大气更宁静、自由大气视宁度小、大气透过率高和大气边界层低等显著优点。并且,在极夜的时候,南极望远镜可以实现数月的连续观测,非常适合时域天文学的研究。迄今为止,中国南极天文中心已经在冰穹A安装了中国之星小望远镜阵和两台南极巡天望远镜。

为了在南极点附近大天区搜寻太阳系外行星,中国科学院南京天文光学技术研究所和中国科学技术大学合作完成了南极亮星巡天望远镜(BSST)的研制,相关研究成果发表在Chinese Optics Letters 2015年第11期上 (Z.Y. Li et al.,Optical design for Antarctic Bright Star Survey Telescope)。

这台望远镜具有300 mm口径和4.8°对角线视场,并含有7个滤光片,观测波段为0.36~1.01 μm。望远镜在国内台址测试,在南极低温环境下使用,工作温度变化范围为40~-80℃,将会面临2个技术难点:在南极使用时,当环境温度快速升高的时候镜面会结霜;温度变化大会产生系统的热变形,尤其是光学元件的热变形,会降低光学成像质量。

该项工作包括BSST光学设计、误差分析和准直装调的过程。作为科研成员之一,李正阳博士表示,基于对南极望远镜建造的深入了解,BSST光学设计和误差分析很好保障了望远镜的成功建造。在晴朗夜晚的试观测结果显示,120秒曝光的星图深度分别为B波段16.7星等、V和R波段17.1星等;观测12星等的目标星,望远镜的测光精度为4.5毫星等。试观测过程中成功观测了HAT-P-3b和HAT-P-12b两个系外行星源。

南极亮星巡天望远镜预计2016年1月安装在南极中山站,在2~3年成功观测后安装到冰穹A。天文学家们无比期待利用BSST这一独特且强大的望远镜对南极上空进行进一步探索。

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