Introduction of Chinese Antarctic Optical Telescopes

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1 Introduction of Chinese Antarctic Optical Telescopes Zhengyang Li* a,b,c, Xiangyan Yuan a,b,c, Xiangqun Cui a,b,c, Lifan Wang d,c, Zhaohui Shang e,c, Fujia Du a,b,c, Xuefei Gong a,b,c, Bozhong Gu a,b,c, Yi Hu e,c, Peng Jiang f,c, Xiaoyan Li a,b,c, Haiping Lu a,b,c, Bin Ma e,c, Fuhai Wei f, Haikun Wen a,b,c, Jin Xu a,b,c, Shihai Yang a,b,c, Honyan Zhou f,c a National Astronomical Observatories / Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing , China; b Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing , China; c Chinese Center of Antarctic Astronomy, Nanjing , China; d Purple Mountain Observatory, Chinese Academy of Sciences Nanjing , China; e National Astronomical Observatories, Chinese Academy of Sciences, Beijing , China; f Polar Research Institute of China, Shanghai , China ABSTRACT The site testing shows that Antarctic Dome A is one of the best site on earth for astronomical observations, for wavelength ranging from visible to infrared and sub-millimeter. Continuous observation for nearly four months in polar nights makes Dome A quite suitable for time domain astronomy. In the past decade CCAA already led a series of Antarctic astronomy activities and telescope projects which will be introduced in this paper. The first generation telescope is Chinese Small Telescope Array known as CSTAR, which was composed of four identical telescopes with 145mm entrance pupil, 20 square degrees FOV and different filters, all pointing to the celestial South Point, mainly used for variable stars detection and site testing. The telescope was deployed in Dome A in Jan. 2008, and followed by automatic observations for four consecutive winters. Three Antarctic Survey Telescopes (AST3) is the second generation telescope capable of pointing and tracking in very low temperature, with 500mm entrance pupil, 8.5 square degree FOV. AST3-1 and AST3-2 were respectively mounted on Dome A in Jan and 2015, fully remotely controlled for supernovae survey and exoplanets searching. In Aug. 2017, AST3-2 successfully detected the optical counterpart of LIGO Source GW Now AST3-3 is under development for both optical and near infrared sky survey by matching different cameras. Based on the experience of the above smaller sized optical telescopes, the 2.5m Kunlun Dark Universe Survey Telescope (KDUST) was proposed for high resolution imaging over wide field of view. Currently the KDUST proposal was submitted to the government and waiting for project review. Keywords: Optical telescopes, Antarctic astronomy, optical design 1. INTRODUCTION The Antarctica plateau is widely considered to be an excellent astronomical site for the superb seeing conditions. Lawrence et al. (2004) have shown that the Dome C (3233m elevation) has a mean seeing value of 0.27arcsec above 30m from the ground [1]. Site testing of Dome A (4091m elevation), led by the Chinese Center of Antarctic Astronomy (CCAA) has shown that the highest point on the Antarctic plateau has a median thickness of the boundary layer was 13.9m from the ice surface with the 25 th and 75 th percentiles at 9.7m and 19.7m [2], and has excellent optical sky backgrounds and large clear-sky fractions (>90%) in the winter months [3]. The monitoring data for wind speed though 2years show that average value is around 4m/sec, 1.2% larger than 10m/s though a whole year. On account of the cold (average -56, lowest -82 ) dry and stable atmosphere above the ground, it also provides exceptional conditions for infrared observations, considered as the best sites for infrared observations on Earth. In polar nights, the in-situ Antarctic telescopes manage to continuous observe for three months, which is quite suitable for the transits detection and time domain astronomy [4]. At Dome C (France-Italian Concordia station), ASTEP (Antarctic Search for Transiting Exo-Planets) South, a fixed 100mm, FOV refractive telescope, was installed in 2008 [5]. ASTEP 400, a 400mm, 1 1 FOV Newtonian telescope equipped with a 5 lens corrector, was installed in Dec. 2009, had been operated from 2010 to 2013, and in The telescope had successfully found 43 planetary transit candidates [6], detected 673 eclipsing *zyli@niaot.ac.cn; phone ; fax ; niaot.ac.cn Ground-based and Airborne Telescopes VII, edited by Heather K. Marshall, Jason Spyromilio, Proc. of SPIE Vol , L 2018 SPIE CCC code: X/18/$18 doi: / Proc. of SPIE Vol L-1

2 binaries and 1166 variable stars [7]. Then a wide FOV 2.5 m optical/infrared telescope had been proposed for Dome C observation with high spatial resolution [8, 9]. Dome A was firstly visited by the 21 st Chinese Antarctic Research Expedition (CHINARE), in Jan Then Kunlun station, 7.9km away from the summit, was built in 2009, is still not a perennial station for now. In partnership with international collaborators, several unattended Chinese wide field Antarctic telescopes have been deployed. The first generation is Chinese Small Telescope Array known as CSTAR monitored an area of 20deg 2 around the South Celestial Pole, which is composed of four identical telescopes with 145mm entrance pupil, deployed on Antarctica in 2008 and its automatic operation continued for four consecutive winters. The fixed telescopes carried out high-cadence time-series observations, and its light curves of non-transient sources have excellent photometric qualities, limited by photon noise and atmospheric scintillation [10]. After correction of the images, the photometric precision reached 4mmag. And 10 promising exoplanet candidates were found [11] and 188 variable stars were identified [12, 13]. In 2012, the telescopes were taken back for upgrade. Following CSTAR, the second generation is three Antarctic Survey Telescopes AST3, with 500mm entrance pupil, 8.5 square degrees FOV, was designed for multi-band surveys and can accurate pointing and tracking [14]. AST3-1 and AST3-2 were respectively mounted on Antarctic plateau in 2012 and 2015, for supernovae survey and exoplanets search. AST3-1 commissioning survey results, consisting of 14 thousand images and 2 million light curves, has been released though the Chinese Astronomical Data Center [15]. In one field of DR1, 339 previously unknown variable stars were detected with i-magnitude less than 16.5 mag during eight days of observations [16]. During the observing season of , the unattended AST3-2 firstly realized all-season wintering observation, automatically worked on exoplanets searching, and found more than 200 promising candidates (paper preparing). In Aug. 18 th 2017, AST3-2 successfully observed the gravitational wave source (GW ), its data revealed a fast evolving transient at ~1 day after the GW trigger [17]. Now, the third one is under construction for both optical and near infrared sky survey by equipping different cameras. The near infrared (NIR) camera is designed to conduct the Kunlun Infrared Sky Survey which will provide a comprehensive exploration of the time varying Universe at the Kdark band (2.4μm) [18]. Another medium size Antarctic optical telescope named Bright Star Survey Telescope BSST had been deployed at Chinese perennial Zhongshan station in Prydz Bay. During the test observation in Aug. 2016, we collected highcadence light curves of Proxima Centauri and detected a tentative transit event [19]. The telescope will join the ongoing survey at Kunlun station in The next generation Chinese Antarctic optical telescope is Kunlun Dark Universe Survey Telescope KDUST, one of two major facility of Chinese Antarctic Observatory which has been listed as National large research infrastructure during twelfth five-year plan. KDUST, a 2.5-meter optic/infrared telescope, will adopt an innovative optical system, which can deliver very good image quality over a 2 square degree flat field of view, and will be perched on a 15-meter-high tower to lift it above the turbulence layer [20]. The proposal was submitted to the government and waiting for project review. 2. CHINESE ANTARCTIC TELESCOPES Generally, there are three critical problems for Antarctic optical telescopes: 1) The telescopes are remotely operated at a temperature ranging from -40 to -80 at Kunlun station, while they are built at 20. The optical performances would have degraded as a result of the temperature deformation, and the electric devices could possibly be damaged due to the extreme cold weather. 2) The lens and mirrors will be frosty when the ambient atmosphere warms up dramatically in Antarctica, and resulted in extinction. The mechanism would possibly be frozen and work improperly. 3) Under the extreme circumstances, to assemble the unattended telescopes and energy module, the expeditions are always short of manpower or time. Besides, the average work duration at Kunlun station was 20days/year, Astronomers will wait for a whole year once the Antarctic telescopes is out of service. Proc. of SPIE Vol L-2

3 2.1 Chinese Small Telescope Array (CSTAR) Figure 1. (left) CSTAR mounted at Kunlun station in 2008; (right) Optical layout of CSTAR CSTAR had been successfully deployed on Kunlun station by 24 th Chinese expedition team in Jan According to the CSTAR design requirement, this telescope has an equivalent aperture of 100mm with wide field of view about 4 and fast f/ratio about 1.2. Four filters from 400nm to 900nm are used for multi-color photometry. The selected detector is ANDOR DV435 1Kx1K frame transfer CCD with pixel size of 13µm. The telescope is fixed without any moving parts, including the focal plane. For such a compact telescope with fast f/ratio and wide field, the catadioptrive objective with spherical primary mirror is selected for our purpose which can deliver lower chromatic aberration. Image quality requires 80% light energy encircled in 2pixels. In order to keep the focus unchanged in about 100 centigrade difference from 20 to -80 centigrade, both the optics and the structure need to select low thermal expansion materials. Zerodur and fused silica are selected for CSTAR s main optical components. The main specifications of the optical system are as follows: Entrance pupil diameter: 145mm (Effective aperture: 100mm) Focal length: 170mm Field of view: 4.48 x 4.48 Working wavelength: g, r, i filter, and one open filter from 400nm to 900nm. 2.2 Antarctic Survey Telescopes (AST3) Comparing with the traditional Schmidt telescope, the optical system of AST3 is compact suitable for transportation, and with a smaller primary mirror, a planar distortion-free focal plane, and can deliver excellent image quality over the whole FOV. According to the science requirement and the selected Camera, the main specifications of AST3 optical system are as follows: 1) diameter of entrance pupil is 500mm; 2) diameter of primary mirror is 680mm; 3) focal ratio is 3.73; 4) the field of view is 4.14 degrees; 5) the working wavelength is nm, with a G ( nm), R ( nm) and I ( nm) filter. Each of the compact telescopes of the AST3 is characterized by good image quality, a planar focal surface, reduced atmospheric dispersion and eliminated distortion. In any wavelength band of R and I, 80% of the light energy of each image spot is within 1 arcsec (observation image quality of AST3-1: the 5σ limiting magnitude is i 18.7 in 60 s with a typical FWHM of 3.7 arcsec and dramatically deepens to i 19.3 with a sharper FWHM of 2.7 arcsec), the largest distortion is 0.012%, less than one pixel of the CCD [21]. oblate primary mirror Transparent aspheric plate Figure 2. Optical layout of AST3 Proc. of SPIE Vol L-3

4 AST3-1 and AST3-2 were respectively mounted on Antarctic plateau in 2012 and 2015, for supernovae survey and exoplanets search. During the austral winter 2017, AST3-2 nicely accomplished the observation, and took data for over 95% of the winter time. The entire observatory is controlled remotely using the Iridium satellite system for transmitting the operational commands. From August 18 th through 28 th AST3-2 observed the first binary neutron star merger system detected by LOGO and Virgo detector network via gravitational wave detection. The AST3-2 data revealed a fast evolving transient at 1 day after the GW trigger, with it i-band magnitude fading from 17.23±0.13 to 17.72±0.09 after 1.8h. Fitting the AST3-2 data, about ~10-2 solar mass of radioactive material was ejected during the merger up to velocities as high as 30% the light speed [17]. Figure 3. Astronomical telescopes at Kunlun station (2017), white box demonstrate the sketch of AST3-3 Figure 4. The red squares mark the position of the kilonova associated with GW [17] 2.3 Antarctic Bright Star Survey Telescope (BSST) BSST, which mainly aims at exoplanets searching by transits detection with a photometric precision less than 0.6% magnitude, has a 300mm aperture, a f-ratio of 2.76 and a FOV of 4.8 degrees ( degrees), and 7 filters (open band, three wide band filters of B,V,R band, three narrow band filters of H-α, Η-β and O-3 band), which meet a wavelength Proc. of SPIE Vol L-4

5 coverage from 0.36um to 1.01um. A 4K 4K and 12μm/pixel CCD camera from ANDOR Corporation is used for imaging of 3arcsec/pixel. The optical performance for central 30arcmin FOV requires 80% energy encircled within 2 pixels, and for the whole FOV 80% energy encircled within 5 pixels. Especially, BSST requires qualified operation in both domestic sites and Antarctic sites. - Figure 5. BSST and its optical layout In April 2015 the telescope had accomplished test observation in Gaomeigu station (Yunnan Observatory). The photometric results show that BSST is able to image a target of 17.1 magnitudes with a SNR (signal to noise ratio) value of 20. Moreover, the median FWHM (full width at half maximum) value is 1.7pixels of the central FOV and less than 2.6pixels of the entire FOV. The photometric precision is superior to 0.4% magnitudes which is measured in transit detection [22]. Figure 6. BSST image of LMC, Observed at Zhongshan Station (Courtesy Jiang Peng & Planis Studio) Proc. of SPIE Vol L-5

6 In 2016, we deployed the BSST at the Chinese Antarctic Zhongshan station, and collected high-cadence light curves of Proxima Centauri in 2016 August and September using the BSST. We detected a tentative transit event at the epoch of 2,457,640.2 HJD, which is compatible with the ephemeris of the RV orbit. After correcting the time-correlated noise, the tentative transit signal reported here has a confidence level of 2.5σ [19]. 2.4 Antarctic Kunlun Dark Universe Survey Telescope The Kunlun Dark Universe Survey Telescope is a 2.5 meter optical/infrared telescope proposed as the next generation Antarctic telescope. Benefit from the treasurable experience of CSTAR AST3 and BSST, KDUST is one of two major astronomical facilities of Chinese Antarctic Kunlun Observatory, which has been listed as National Large Research Infrastructure during 12 th Five-year plan. The proposal has already submitted to the government and under reviewing. Figure 7. Sketch of KDUST (left: KDUST on a 15m tower; right: optical layout) The preliminary design of KDUST optical system is a coaxial three-mirror anastigmatic telescope system with a 2.5-m F/1.06 primary mirror. The innovative optical system, originated from the coude system of Chinese 2.16 meter telescope. With an aspheric tertiary mirror KDUST can deliver qualified image quality over a 2 square degree FOV. The model and optical layout are shown in Figure 7. The required image quality measured by the 80% encircled energy is less than 0.3 arc sec in accordance with the free seeing parameter of Antarctic Dome A. The F/1.06 primary mirror can guarantee a short tube and large back focal length to simplify the arrangement of the focal instruments, and the instruments will remain motionless during pointing or tracking. To make full use of the superb atmospheric seeing, KDUST will be mounted on a tower with a height of 15meter [23]. 3. CONCLUSION This paper briefly introduced Chinese Antarctic optical telescopes from small fixed telescope to large aperture optical/infrared telescope, including CSTAR, AST3, BSST and KDUST. Though all of them are used for wide field imaging, they have different optical systems for different main science goals. Based on the experience of the Antarctic telescope development and operation, high resolution telescope over wide of view will be our next direction of effort in order to fully use the unique advantages of Antarctic Dome A for the very competitive science. 4. ACKNOWLEDGEMENT The authors would like to thank the Chinese Arctic and Antarctic Administration and Polar Research Institute of China for supporting the Antarctic Astronomical Research Expedition. And the authors are grateful for the support of the National Natural Science Foundation of China ( , ), and the National Key Basic Research Program of China ( CB834901). The authors appreciate Tsinghua University and Nanjing University for their financial support, and appreciate Prof. Michael Ashley for his great work of developing the PLATO and PLATO-A. Proc. of SPIE Vol L-6

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