Noname manuscript No. (will be inserted by the editor)

The Bright Survey Telescope for the Planetary Transit Survey in Antarctica

Qiguo Tian · Peng Jiang · Fujia Du · Jian Wang · Zhengyang Li · Xiaoyan Li · Zhiyong Zhang · Haiping Lu · Xiangyan Yuan · Huigen Liu · Hui Zhang · Luming Sun · Liang Chang · Jianguo Wang · Shaohua Zhang · Tuo Ji · Xiheng Shi · Jie Chen · Guangyu Zhang · Minghao Jia · Jiajing Liu · Junyan Zhou · Xiang Pan · Shucheng Dong · Fengxin Jiang · Hongfei Zhang · Jilin Zhou · Lifan Wang · Hongyan Zhou

Received: date / Accepted: date

Qiguo Tian Polar Research Institute of China, Shanghai 200136, China

Peng Jiang Polar Research Institute of China, Shanghai 200136, China School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China E-mail: [email protected] Fujia Du National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China

Jian Wang State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Zhengyang Li National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China

Xiaoyan Li National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China

Zhiyong Zhang National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China 2 Tian et al.

Haiping Lu National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China

Xiangyan Yuan National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technol- ogy, Chinese Academy of Sciences, Nanjing 210042, China

Huigen Liu School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China

Hui Zhang School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China

Luming Sun Polar Research Institute of China, Shanghai 200136, China Key Laboratory for Research in and , Department of Astronomy, Uni- versity of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, China

Liang Chang National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, P.O. Box 110, Kunming, Yunnan 650011, China Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, Yunnan 650011, China

Jianguo Wang National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, P.O. Box 110, Kunming, Yunnan 650011, China Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, Yunnan 650011, China

Shaohua Zhang Polar Research Institute of China, Shanghai 200136, China Tuo Ji Polar Research Institute of China, Shanghai 200136, China Xiheng Shi Polar Research Institute of China, Shanghai 200136, China Jie Chen State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Guangyu Zhang State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China BSST 3

Minghao Jia State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Jiajing Liu State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Junyan Zhou Polar Research Institute of China, Shanghai 200136, China Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, Uni- versity of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, China

Xiang Pan Polar Research Institute of China, Shanghai 200136, China Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, Uni- versity of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, China

Shucheng Dong State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Fengxin Jiang State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Hongfei Zhang State Key Laboratory of Technologies of Particle Detection and Electronics, Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China

Jilin Zhou School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China

Lifan Wang Purple Mountain Observatory, Chinese Academy of Science, Nanjing 210008, China

Hongyan Zhou Polar Research Institute of China, Shanghai 200136, China Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, Uni- versity of Science and Technology of China, Chinese Academy of Sciences, Hefei 230026, China 4 Tian et al.

Abstract Transiting extrasolar planets (), especially those orbiting bright , are desired for study of the diversity of planetary compositions, internal structures and atmospheres beyond our Solar System. Dome A at Antarctica is a promising site for planetary transit surveys, where the contin- uous darkness and the large clear–sky fraction in the winter months greatly en- hance the detection efficiency. The Chinese Small Telescope ARray (CSTAR) and the Antarctic Survey Telescopes (ASTs) are the first facilities that have been operated at Dome A for use in surveys. To increase the sky coverage, a low–temperature–resistant wide–field robotic telescope, named the Bright Star Survey Telescope (BSST), has been developed to join the ongo- ing planetary transit survey in Antarctica. The BSST has an aperture size of 300 mm and is equipped with a large–frame 4K × 4K CCD camera to receive starlight from a 3.◦4 × 3.◦4 field of view. The BSST was operated at Lijiang observatory in April and May 2015 for a test run. Photometric precision of 3.5 mmag was achieved for stars with V ∼ 11 mag using 75 s exposures. The transiting events of two –size exoplanets, HAT–P–3b and HAT–P–12b, were observed on May 10 and May 20, 2015, respectively.

Keywords Instrumentation · Exoplanet · · Antarctic Site

PACS 95.55.Cs · 97.82.Cp

1 Introduction

The discovery of exoplanets has been one of the remarkable developments in astronomy over the last two decades. The first planetary companions to solar–type stars were inferred from the observed periodic Doppler reflex motion of primary stars using a (RV) technique in the 1990s[1,2]. An exoplanet transiting event was first observed in the HD 209458b system[3] in 2000. The existence of transiting exoplanets allows us to measure both the planetary mass and the planetary radius. These measurements provide important clues about the formation and evolution of planetary systems. To date, more than 200 transiting exoplanets have been discovered by ground–based wide–field photometric surveys (e.g., HATNeT[4]; superWASP[5]; KELT[6]). The Kepler mission, which is capable of continuous measurement of the brightness variations of about 100,000 solar–type stars with an unprece- dented accuracy in the order of 20 ppm (parts per million) in a single field of approximately 105 square degrees, found 4696 transiting planetary candi- dates, 1028 of which have been confirmed to date. However, only 168 Kepler transiting planets, for which the host stars are at the top of brightness dis- tribution, have had their masses determined using the RV technique. These planets, combined with the transiting planets discovered by ground–based sur- veys, compile a sample of ∼350 exoplanets with both the masses and radii of these planets are known. In order to study the diversity of planetary compo- sitions, internal structures and atmospheres[7,8], more transiting exoplanets BSST 5

around bright stars are needed since they are ideal targets for detailed spec- troscopic follow–up characterizations. Several surveys have been designed for this purpose, such as the Transiting Exoplanet Survey Satellite (TESS[9]), the CHaracterising ExOPlanet Satellite (CHEOPS[10]) and the Next–Generation Transit Survey (NGTS[11]).

The detection efficiency of ground–based transiting planet surveys at mid– latitude observatories, especially for planets with orbital periods of tens of days, is seriously limited by the diurnal cycle that breaks the observation campaigns[12]. Dome A[13,14], Antarctica, which naturally offers continuous darkness during its winter[15], can significantly enhance the detection effi- ciency of a transiting planet survey by a factor of two to three[16]. Moreover, the atmospheric scintillation at Dome A is predicted to be much weaker than that observed at mid–latitude observatories[17,18], thus providing a potential gain of >2 in photometric precision. Therefore, Dome A is a promising astro- nomical site for transiting planet surveys. In 2008, a pioneering astronomical facility called the Chinese Small Telescope ARray (CSTAR)[19] was installed at Dome A. Its observation is fixed on a 4.◦5 × 4.◦5 sky area around the South Celestial Pole. Between March 4 and August 8, 2008, CSTAR collected 10,000 high–cadence high–precision light curves of bright stars using an observation time of 1728 hours[20]. Ten transiting exoplanet candidates were identified[21]. In addition, the quality of the CSTAR images suggests that a large percentage (>90%) of the available time was suitable for high–precision photometry[22] in the observing season.

The successful operation of CSTAR has encouraged us to conduct a tran- siting planet survey while taking advantage of the good observation conditions at Dome A. Our goal is to detect gas/ice giant planets with orbital periods of <50 days around stars with brightness higher than V = 13 mag in a large sky area around the South Celestial Pole. During the 28th and 31st Chinese National Antarctic Research Expeditions, two Antarctic Survey Telescopes (ASTs) with aperture of 500 mm were installed at Dome A. Exoplanet survey represents only one of the main scientific goals, and the ASTs are obliged to spare a large fraction of telescope time to observe targets in other astronomical fields. In order to increase the sky coverage of the planetary transit survey, a wide–field photometric telescope, named the Bright Star Survey Telescope (BSST), has been developed. The overall technical specifications of the BSST are summarized in Table 1. The BSST was installed at Lijiang observatory, Yunnan, in April 2015 and the test observation run lasted for two months. In this paper, we report the telescope design and its on–sky performance. The paper is organized as follows. In the next section, we present the telescope design. We report on the early on-sky performance testing of the BSST in section 3, and our future plans are discussed in section 4. 6 Tian et al.

2 Instrument Design

2.1 Optical and Mechanical Systems

A schematic diagram of the cross section of the Ritchey–Chretien (R–C) opti- cal configuration of the BSST is presented in Fig. 1. The configuration mainly consists of a primary mirror (M1), a secondary mirror (M2), and corrector lenses, with diameters of 315 mm, 113 mm and 324 mm, respectively. The R–C optical design achieves a large field of view (FOV) of 11.5 square degrees, wide wavelength coverage ranging from 0.36 to 1.014 µm, and good image quality, and allows easy CCD installation[23]. To minimize image quality deterioration due to the huge difference between the temperature in Antarctica and that in the manufactory, we have used low Coefficient of Thermal Expansion (CTE) materials for the optical system. Zerodur (CTE: 00.05 ppm K−1) is used for the reflective mirrors, and fused silica (CTE: 0.55 ppm K−1) is used for the corrector lenses. The telescope tube is sealed using a flat enclosure window to prevent the occurrence of frosting on the surfaces of these glasses due to severe temperature variations, which can seriously impair both the throughput and the image quality[19]. The enclosure window is coated with a layer of Indium- Tin-Oxide (ITO) film, which can be heated by an electrical current, to keep the surface slightly warmer than the ambient, and thus effectively keep itself from frosting and icing. The telescope tube is carried by a commercial German mount (AP 1600GTO manufactured by Astro–Physics, Inc. United States). A customized dovetail is installed on top of the DEC plate and the tube is assembled on this dove- tail. The focusing mechanism is realized by utilizing a ball screw drive system. The filter wheel is driven directly by a stepper motor, delivering an angular precision of ≤ 1′′. We have developed a telescope control system to run the telescope remotely. It consists of a pointing (RA and DEC) controller, an ab- solute encoder controller, and a focusing and filter controller. All the electronic devices, along with a control computer, are placed in a thermal–control case. Since the moving parts and electronics of the telescope must be main- tained at a temperature above the lowest operating temperature (−30◦C) to function properly, we wrap the encoder, the motor and the electronics with temperature–resistant materials. In addition, heaters controlled by a Pro- grammable Logic Controller (PLC) can be powered up to warm the devices once their local temperature is lower than −30◦C. In order to ensure telescope operation in the harsh environment at Dome A, low–temperature resistance tests have been performed on the subsystems in a temperature–controlled cab- inet. The mount initially keeps static when the temperature in the cabinet is gradually reduced to −70◦C. The heaters effectively ensure that the devices are warmer than −30◦C, while the cabinet temperature is lower. We then in- crease the temperature step by step, and successfully run the mount over the temperature range from −60◦C to 50◦C. The focusing and filter system also passed these low–temperature resistance tests. BSST 7

2.2 Camera System

The camera is a custom–designed Andor iKon–XL CCD camera. The CCD sensor is a CCD203–82 with a basic broadband coating, produced by E2V Technologies. The sensor is arranged as a 4096 × 4096 array with a pixel size of 12 µm × 12 µm. The camera is equipped with a mechanical shutter, for which the operating temperature must be above 0◦C. A thermal–control shut- ter housing was thus developed to warm the shutter when necessary. Titanium alloy with low thermal conductivity was used to build a thermally–insulating cavity, which is additionally wrapped in an insulating material. There are two circular heater plates are mounted on the upper and lower sides of the cavity. Two temperature sensors are installed to monitor the temperature inside the cavity. The shutter housing thus operates in a closed loop feedback manner, providing temperature accuracy of 0.1◦C. The shutter housing was tested in a deep-cooled refrigerator. It takes 13 minutes to heat the cavity from −80◦C up to 5◦C, and the mechanical shutter works properly in the experiment.

2.3 Remote Control System and Data Storage

The remote control system is divided into four layers[24]. The bottom layer is the device layer. Above that is the device controller layer, including the telescope control system, the dome controller, the camera control system, the meteorological station, and the data processing module. The observation con- trol layer is used to control the entire telescope system. The user layer pro- vides two user interfaces. A local user interface and a remote user interface, that operates via a satellite communication channel for observers to access the telescope remotely during operation at Dome A. There are a total of eight hard drives, each with a capacity of 1 TB, in the data storage system and these drives are divided into four groups. Each group consists of two hard drives, forming a RAID1, and is bundled in an enclosure. The total capacity is thus 4TB, which is sufficient to store the images acquired in two observing seasons. To enhance the system reliability, each enclosure is equipped with two different types of hard drive: a mechanical drive and a solid-state drive. We also developed a hard drive power management module to relieve power consumption issue at Dome A. The power supplies of enclosures can be switched on or off via the disk power management module. Only one disk enclosure is switched on for recording the images during observation.

3 On–Sky Performance

The BSST was installed at Lijiang observatory in April 2015 and was operated there for two months. An analysis of the telescope’s on–sky performance is presented below. 8 Tian et al.

3.1 Image Quality

Figure 2 shows the stellar point spread functions (PSFs) across the FOV in a typical image. Overall, the image quality is acceptable. According to the optical design, the minimal full width at half maximum (FWHM) is located at the center and the FWHM increases outwards. However, the results show some differences. The position with the best quality on the image is located on the right, where the FWHM is <1.8 pixels, rather than at the center, where the FWHM is ∼2 pixels. While the left edge of the image shows the worst quality, with a FWHM >3 pixels. The cause of this phenomenon has been identified to be the misalignment between CCD camera and its support platform, which is difficult to fix on–site. The support platform has thus been modified to improve the image quality.

3.2 Differential Photometric Precision

The BSST is designed primarily for acquiring differential time–series pho- tometry data of bright stars. The photometric performance of the BSST is examined by measuring the light curve scatters as a function of the star mag- nitudes. We used images of the WASP–43 field observed on April 20, 2015 to study the differential photometric precision. The night was mostly clear, but humidity remained fairly high. The average wind speed was about 4.5 m s−1 and the maximum instantaneous wind speed was 10 m s−1. The observation conditions were not perfect for high precision photometry, but this was the best night that we encountered during the test run. The rainy season came 1 month earlier than those in previous years at the Lijiang observatory, and thus there were far fewer clear nights than we had expected when planning the run. The WASP–43 field was observed with an exposure time of 25 s and the observation lasted for 4 hours. We combined three consecutive frames at a time to produce images with an effective exposure time of 75 s. These com- bined images were reduced using our customized data pipeline (Sun L. M. in preparation). In Fig. 3, we plot the distribution of the photometric scatters versus the V band magnitudes for 645 stars with 10.5 < V < 16.5 mag in the center field. For stars with V > 14 mag, the observed scatters are consistent with the expected error arising from photon noise and sky background noise. For bright stars with V ∼ 11 mag, the observed photometric scatters are about 3.5 mmag, where the photon noise is never the dominant error source. More sources of errors have to therefore be considered in the error budget. Atmospheric scintillation is an important noise source for photometry with small aperture telescopes[25]. Its amplitude on typical photometric nights is given as ( ) X3/2 h σ = 0.09 √ exp − D2/3 2t 8 , where X is the airmass, D is the telescope diameter in cm, t is the exposure time in s, and h is the altitude of the observatory in km. We estimate the BSST 9 atmospheric scintillation to be 1 mmag for the combined images of the WASP– 43 field. The readout noise and the dark current noise are extremely small compared with the expected scintillation, and thus they are negligible in the error budget. However, the flat–field noise measured in the master calibration image is 0.2% per pixel, which is not ideal. The camera shutter equipped on the BSST takes about 60 ms to close completely, and we found that the shutter effect was significant in flat–field images with exposure time of less than 2 s. The flat–field images with longer exposure time are not satisfactory either, because too many stars appear. Therefore, we decided to use flat–field images with 2 s exposure time. Since the ideal sky brightness for a 2 s exposure time only lasts for a few minutes per day, our co–added flat–field image is limited to a signal–to–noise ratio of 500. The flat–field noise for a specific star can be estimated as an average over the brightest pixels in its aperture. For the BSST, the core of a star is generally sampled by 4 pixels, and thus the flat– field noise is about 1 mmag. Note that it is a conservative estimation as the flat–field noise in the common pixels of consecutive frames for a star would cancel out. By considering the theoretical atmospheric scintillation and the flat–field noise, the photometric precision limit is about 1.4 mmag. It is still not sufficient to account for the observed 3.5 mmag photometric scatters of the bright stars in the WASP-13 field. Since the observation conditions were not perfect during the night, we suspect that fluctuations of the transparency over the wide field of the BSST may cause additional noise. We derived the average transparency curve by combining the light curves in the instrumental magnitudes of bright stars. We found that 20% of the images suffered from higher extinctions, by a level of 0.1 mag, than the derived from the airmass. These images might be affected by thin high–altitude clouds. After removing these images, we achieved a differential photometric precision of 2.9 mmag for the bright stars. We also suspect that the atmospheric scintillation may be much larger than the theoretical value since the wind speed was high and the weather conditions were not stable over the observation night. Our current goal is to discover gas/ice giant planets. The photometric pre- cision that the BSST achieved at the Lijiang observatory is satisfactory. In order to improve the precision, we have to create high signal–to–noise ratio flat–field images. Superflat corrections based on night sky[26] and photometric superflat corrections[27] are two alternative flat–fielding methods, and neither of which requires traditional flat–field images on a bright sky. They are very useful for observations at Dome A during polar nights. We also have to update our data pipeline to deal with systematic errors that would possibly appear while approaching higher precision. The atmospheric scintillation during win- ter time at Dome A, where the turbulent boundary layer is thin[28] and the high–altitude wind speed is low[18], is expected to be lower than that of mid– latitude observatories[17], suggesting a potential gain of >2 in photometric precision. In addition, the transparency at Dome A is definitely the best on the ground. Therefore, we expect to achieve 1 mmag photometry precision for 10 Tian et al.

bright stars when the BSST begins observations at Dome A. It will enable us to discover transiting exoplanets of SuperEarth–size or less.

3.3 Photometric Sensitivity

In addition to searching for planetary transits around bright stars in a survey mode, we also plan to observe targets of specific interest in a pointing ob- servation mode with a small fraction of the telescope time. Three broadband filters (standard Bessel B and V filters and a customized “Red” filter with a central wavelength of 760 nm and bandwidth of 250 nm) have been designed for pointing observations. During the test run in Lijiang, we observed several active galactic nuclei (AGNs) with the BSST to study their variability[29]. The images of these faint targets are then used to characterize the photo- metric sensitivity of the BSST in the B and V bands. Since all the AGNs observed in Lijiang were selected from the (SDSS), we conveniently used the SDSS photometric catalog for flux calibration[30,31]. The non–variable stars in the field are carefully selected as the flux calibration stars. Their B and V magnitudes are then converted from the PSF magnitudes of the SDSS u, g, r and i bands by following the procedure of Jester et al.[32]. For a typical exposure time of 2 minutes, the magnitude limits are B = 16.7 and V = 17.1 mag (detections with a signal–to–noise ratio of 10), respectively.

3.4 Transits of HAT–P–3b and HAT–P–12b

In order to demonstrate the Jupiter–sized transiting planet detection capa- bility of the BSST, we have observed two known transiting planets: HAT–P– 3b[33] on May 10, 2015 and HAT–P–12b[34] on May 20, 2015. Analysis of the light curve parameters is beyond the scope of this paper. HAT–P–3b is a Jupiter–size planet transiting a K dwarf star (GSC 03466– 00819; V = 11.56 mag) with a period of ∼2.9 days. Its transit duration is ∼2 hours and the transit depth is ∼15 mmag. The transit was observed using the open band with a unified exposure time of 10 seconds in a cadence of 18 seconds. The observation was interrupted for 10 minutes at the mid–transit due to clouds, and 551 useful frames were finally acquired in 3.3 hours. We intended to cover the full light curve of HAT–P–3b. However, the observed start of the transit ingress was about 50 minutes later than the ephemerides reported in the discovery paper[33]. Therefore, the end of the transit egress was not covered. We updated the ephemerides of HAT–P–3b using the data collected by the Exoplanet Transit Database (ETD)1. The observed event is consistent with the updated ephemerides within 3 minutes. The precision of differential photometry is about 10 mmag for HAT–P–3 and a nearby reference star with similar brightness (see Fig. 4). The high precision light curve (∼1 mmag) of the transit in the R band acquired by Nascimbeni et al.[35] was overplotted

1 http://var2.astro.cz/ETD/ BSST 11

with our BSST measurements for comparison 2. The transit depth in the open band is significantly larger than the depth observed in the R band. The difference possibly suggests that the effective radius of HAT–P–3b observed in short wavelengths is larger than that observed in the R band. The change in effective radius as a function of wavelength offers an important clue to understand the planetary atmosphere[36]. HAT–P–12b is a transiting exoplanet orbiting a K4 dwarf star, GSC 03033– 00706 (V = 12.8 mag). The exoplanet has an orbital period of ∼3.2 days and a transit duration of ∼2.3 hours. Its radius is almost the same as that of Jupiter, and it yields a transit depth of ∼25 mmag. We defocused the telescope slightly to allow longer exposures. The observed FWHM of the PSF is 3.5 pixels, larger than the designed FWHM of 2.0 pixels. The target was observed with a cadence of 90 seconds. We used 50 seconds exposure time at the beginning and increased to 70 seconds later. The observation lasted for 3.5 hours and 156 useful frames were acquired. The end of the transit egress was not covered due to thick cloud cover. The differential photometric precision is about 8.5 mmag for both HAT–P–12 and a reference star with V = 12.6 mag. Higher precision of 4.6 mmag is achieved for a brighter reference star with V = 11.45 mag. The light curves observed in the g and z bands by Hartman et al.[34] are overplotted with the BSST curve in Fig. 5.

4 Future Plans

Over the next two to three years, we plan to operate the BSST at the Zhong- ′ ′′ ′ ′′ shan station (south 69◦22 23.36 , east 76◦22 17.14 ) on the Larsemann hills in Prydz Bay, where the supporting facilities are much better. The BSST has arrived at Zhongshan station along with the 32nd Chinese Antarctic Expe- dition. While the BSST operating at Zhongshan station, we will improve the supporting capacity at the Kunlun station, install a third AST, and build two more BSSTs. These BSSTs will eventually be moved to the Chinese Antarctic ′ ′′ ′ ′′ Kunlun station (south 80◦25 01 , east 77◦06 58 ), which is approximately 7.3 km southwest of Dome A. The project will greatly increase the sky coverage and the detection efficiency of the Antarctic planetary transit survey. As part of the Antarctic planetary transit survey, the three ASTs and the three BSSTs will cover a sky area of 300-500 square degrees and produce about 1,000,000 long-term high-cadence high-precision bright star light curves.

Acknowledgements The authors appreciate the enlightening suggestions from the anony- mous referees, which helped to improve the quality of this paper. This work is supported by the Astronomical Project for the Chinese Antarctic Inland Station, the NSFC grant (grant nos. 11233002, 11203022, 11421303, 11473025, and 11033007), the National Basic Re- search Program of China (973 Program, grant no. 2015CB857005, grant No. 2013CB834905), the SOC program (CHINARE2012-02-03, CHINARE2013-02-03, CHINARE2014-02-03, and CHINARE2015-02-03) and the Shanghai Natural Science Foundation (grant no. 14ZR1444100).

2 The updated ephemerides were applied to calculate the phase offset between the two light curves. 12 Tian et al.

Table 1 Technical Specifications of the BSST

Parameter Specification Telescope Aperture 300 mm Field of View 3.◦4 × 3.◦4 CCD Pixel 12 µm Image Scale 3′′ per pixel Image Quality 1.5–3 pixel FWHM over entire field Filter B, V, “Red”; Hα,Hβ,O III]; Open Maximum Speed 2◦ s−1 Pointing accuracy <3′ Tracking accuracy 1.5′′ in 5 min (RMS) Operation temperature −80◦C − 40◦C

Fig. 1 Cross section of the R–C Optics of the BSST. It mainly consists of a primary mirror, a secondary mirror, corrector lenses and a flat enclosure window. The field of view is 11.5 square degrees and the wavelength coverage range is from 0.36 to 1.014 µm. BSST 13

Fig. 2 Representative stellar PSFs, shown as contours of the intensity profiles of unsatu- rated stars and taken with a 5×5 grid pattern on the CCD. Each panel is 10 pixels on a side. Contours show levels of 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, and 100% of the peak intensity. 14 Tian et al.

Fig. 3 Observed scatters as a function of magnitudes for the WASP–43 field with an effec- tive exposure time of 75 s. There are 645 stars with 10.5 < V < 16.5 mag are used for the analysis. The blue dashed line shows the expected scatters from photon noise and the sky background noise, and the red solid line shows the curve fitted to the observed scatters. For the stars with V > 14 mag, the observed scatters are consistent with the expected errors. For the bright stars with V ∼ 11 mag, the observed photometric scatters are about 3.5 mmag, suggesting photon noise is not the dominant error source. BSST 15

Fig. 4 Bottom: light curve for HAT–P–3b observed on May 10, 2015 with BSST. The green points are the high precision light curve acquired by Nascimbeni et al[35]. Off–transit magnitude has been set to zero. Top: Differential photometric precision for a nearby reference star with similar brightness. 16 Tian et al.

Fig. 5 Bottom: light curve for HAT–P–12b observed on May 20, 2015 with BSST. The color points (green: g band; orange: z band) are the high precision light curves acquired by Hart- man et al[34]. Off–transit magnitude has been set to zero. Middle: Differential photometric precision for a nearby reference star with similar brightness. Top: Differential photometric precision for a brighter reference star. BSST 17

References

1. Mayor M, Queloz D (1995) A Jupiter–mass companion to a solar–type star. Nature 378:355–359 2. Marcy G W, Butler R P (1996) A Planetary Companion to 70 Virginis. Astrophys J 464:L147–L151 3. Charbonneau D, Brown T M, Latham D W, Mayor M (2000) Detection of planetary transitsacross a sun–like star. Astrophys J 529:L45–L48 4. Bakos G, Noyes R W, Kov´acsG et al (2004) Wide–Field millimagnitude photometry with the HAT: A tool for extrasolar planet detection. Publ Astron Soc Pac 116:266–277 5. Pollacco D L, Skillen I, Collier C A et al (2006) The WASP project and the SuperWASP cameras. Publ Astron Soc Pac 118:1407–1418 6. Pepper J, Pogge R W, DePoy D L et al (2007) The Kilodegree Extremely Little Telescope (KELT): A Small Robotic Telescope for Large-Area Synoptic Surveys. Publ Astron Soc Pac 119:923–935 7. Fortney J J, Marley M S, Barnes J W (2007) Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation: Application to Transits. Astrophys J 659:1661– 1672 8. Burrows A S (2014) Highlights in the study of exoplanet atmospheres. Nature 513:345– 352 9. Ricker G R, Winn J N, Vanderspek R et al (2014) Transiting Exoplanet Survey Satellite (TESS). Proc of SPIE 9143:914320 10. Broeg C, Fortier A, Ehrenreich D et al (2013) CHEOPS: A transit photometry mis- sion for ESA’s small mission programme. European Physical Journal Web of Conferences 47:03005 11. Wheatley P J, Pollacco D L, Queloz D et al (2013) The Next Generation Transit Survey (NGTS). European Physical Journal Web of Conferences 47:13002 12. Brown T M (2003) Expected detection and false alarm rates for transiting jovian planets. Astrophys J 593:L125–L128 13. Yang H, Allen G, Ashley M C B et al (2009) The PLATO Dome A Site-Testing Obser- vatory: Instrumentation and First Results. Publ Astron Soc Pac 121:174–184 14. Tang X, Sun B, Guo J et al (2015) A freeze-on ice zone along the ZhongshanCKunlun ice sheet profile, East Antarctica, by a new ground-based ice-penetrating radar. Sci Bull 60:574–576 15. Zou H, Zhou X, Jiang Z et al (2010) Sky brightness and transparency in the i band at Dome A, antarctica. Astron J 140:602–611 16. Law N M, Carlberg R, Salbi P et al (2013) Exoplanets from the arctic: the first wide– field survey at 80◦N. Astron J 145:58–68 17. Kenyon S L, Lawrence J S, Ashley M C B et al (2006) Atmospheric scintillation at Dome C, Antarctica: Implications for photometry and astrometry. Publ Astron Soc Pac 118:924–932 18. Hagelin S, Masciadri E, Lascaux F, Stoesz J (2008) Comparison of the atmosphere above the South Pole, Dome C and Dome A: first attempt. Mon Not R Astron Soc 387:1499–1510 19. Yuan X, Cui X, Liu G et al (2008) Chinese Small Telescope ARray (CSTAR) for Antarc- tic Dome A. Proc of SPIE 7012:70124G-1-8 20. Wang S H, Zhou X, Zhang H et al (2014) The correction of diurnal effects on CSTAR photometry. Res Astron Astrophys 14:345–356 21. Wang S, Zhang H, Zhou J L et al (2014) Planetary transit candidates in the CSTAR field: analysis of the 2008 Data. Astrophys J Suppl Ser 211:26–40 22. Ivezi´c Z,˘ Smith J A, Miknaitis G et al (2007) Sloan digital sky survey standard star catalog for stripe 82: the dawn of industrial 1% optical photometry. Astron J 134:973–998 23. Li Z Y, Lu H P, Yuan X Y (2015) Optical design for Antarctic Bright Star Survey Telescope. Chin Opt Lett 13:111101–1–5 24. Zhang G Y, Wang J, Tang P Y et al (2015) An autonomous observation and con- trol system based on EPICS and RTS2 for Antarctic telescopes. Mon Not R Astron Soc 455:1654–1664 25. Dravins D, Lindegren L, Mezey E, Young A T (1998) Atmospheric Intensity Scintillation of Stars. III. Effects for Different Telescope Apertures. Publ Astron Soc Pac 110:610–633 18 Tian et al.

26. Manfroid J (1995) Stellar calibration of CCD flat fielding. Astron Astrophys 113:587 27. Manfroid J, Royer P, Rauw G, Gosset E (2001) Correction of Systematic Errors in Differential Photometry. Astronomical Data Analysis Software and Systems X 238:373 28. Bonner C S, Ashley M C B, Cui X et al (2010) Thickness of the Atmospheric Boundary Layer Above Dome A, Antarctica, during 2009. Publ Astron Soc Pac 122:1122–1131 29. Zhang S H, Zhou H Y, Shi X H et al (2015) Discovery of Extremely Broad Balmer Absorption Lines in SDSS J152350.42+391405.2. Astrophys J 815:113–123 30. Mao Y N, Lu X M, Wang J F (2014) Simultaneous multicolor photometry of fast-moving objects using the 1-meter telescope at Xinglong Observatory. Sci China-Phys Mech Astron 57:562–568 31. Rong J, Buser R (2000) The Galactic population components and their structural pa- rameter values. Chin Sci Bull 45:1362-1364 32. Jester S, Schneider D P, Richards G T et al (2005) The Sloan digital sky survey view of the Palomar–Green Bright Survey. Astron J 130:873–895 33. Torres G, Bakos G A,´ Kov´acsG et al (2007) HAT–P–3b: A Heavy–Element–rich Planet Transiting a K Dwarf Star. Astrophys J 666:L121–L214 34. Hartman J D, Bakos G A,´ Torres G et al (2009) HAT–P–12b: A Low–Density Sub– Saturn Mass Planet Transiting a Metal–Poor K Dwarf. Astrophys J 706:785–796 35. Nascimbeni V, Piotto G, Bedin L R, Damasso M (2011) TASTE: The Asiago Search for Transit timing variations of Exoplanets. I. Overview and improved parameters for HAT–P–3b and HAT–P–14b. Astron Astrophys 527:A85–A94 36. Nascimbeni V, Piotto G, Pagano I et al (2013) The blue sky of GJ3470b: the atmosphere of a low-mass planet unveiled by ground-based photometry. Astron Astrophys 559:A32