Site-testing at Muztagh-ata site Ⅰ:ground meteorology and sky brightness

Site-testing is crucial for achieving the goal of scientific research and analysis of meteorological and optical observing conditions,one of the associated basic tasks.As one of three potential sites to host the 12-meter Large Optical/infrared Telescope(LOT),the Muztagh-ata site,which is located on the Pamir Plateau in Xinjiang,in west China,began its site-testing task in the spring of 2017.In this paper,we firstly start with an introduction to the site and then present a statistical analysis of the ground-level meteorological properties such as air temperature,barometric pressure,relative humidity,and wind speed and direction,recorded by an automatic weather station with standard meteorological sensors for a two-year duration.We also show the monitoring results of sky brightness during this period.

Site testing campaign for the Large Optical/infrared Telescope of China:overview

The Large Optical/infrared Telescope(LOT)is a ground-based 12 m diameter optical/infrared telescope which is proposed to be built in the western part of China in the next decade.Based on satellite remote sensing data,along with geographical,logistical and political considerations,three candidate sites were chosen for ground-based astronomical performance monitoring.These sites include:Ali in Tibet,Daocheng in Sichuan and Muztagh-ata in Xinjiang.Up until now,all three sites have continuously collected data for two years.In this paper,we will introduce this site testing campaign,and present its monitoring results obtained during the period between March 2017 and March 2019.

Long-term analysis of clear nights using satellite data considering astronomical sites in western China

A large ground-based optical/infrared telescope is being planned for a world-class astronomical site in China.The cloud-free night percentage is the primary meteorological consideration for evaluating candidate sites.The data from GMS and NOAA satellites and the MODIS instrument were utilized in this research,covering the period from 1996 to 2015.Our data analysis benefits from overlapping results from different independent teams as well as a uniform analysis of selected sites using GMS+NOAA data.Although significant ground-based monitoring is needed to validate these findings,we identify three different geographical regions with a high percentage of cloud-free conditions(~83%on average),which is slightly lower than at Mauna Kea and Cerro Armazones(~85%on average)and were chosen for the large international projects TMT and ELT respectively.Our study finds evidence that cloud distributions and the seasonal changes affected by the prevailing westerly winds and summer monsoons reduce the cloud cover in areas influenced by the westerlies.This is consistent with the expectations from climate change models and is suggestive that most of the identified sites will have reduced cloud cover in the future.

Site-testing at Muztagh-ata site Ⅲ:temperature inversion in surface-layer atmosphere

In this article,we present detailed seasonal,monthly and daily statistics of temperature difference in the surface layer at the Muztagh-ata site based on the temperature measurements at two heights of 2 m and6 m.We find that temperature inversion occurs frequently at our site during nighttime,especially during the cold season.Strong temperature inversion always represents stable atmospheric turbulence,which is crucial for an optical observatory.By analyzing the behavior of temperature inversion and its correlation with wind and cloud amount,one conclusion can be made that radiation inversion is the main reason for the existence of temperature inversion in the surface-layer at the Muztagh-ata site.

Preface:Site testing campaign for the Large Optical/infrared Telescope of China

The Large Optical/infrared Telescope of China is a 12 meter diameter ground-based optical/infrared telescope which is proposed to be constructed in the western part of China in favor of its arid climate,and clear,dark nights.A site testing campaign was initiated between November 2016 and March2019 in order to investigate long term astronomical weather conditions in the western part of China,specifically,at three sites:Ali in Tibet,Daocheng in Sichuan and Muztagh-ata in Xinjiang.This is the first attempt to evaluate long term performances of three candidate sites with uniformly calibrated instruments and data collecting/processing methods for a two-year span in China.In this special issue,we present nine papers that describe the whole site testing campaign,including preparation,data collecting/processing methods,instrument set up at three candidate sites as well as detailed statistics for the measurement results from March 2017 to March 2019.

Site testing at Muztagh-ata site Ⅱ:seeing statistics

In this article,we present a detailed analysis of the statistical properties of seeing for the Muztaghata site which is a candidate site for hosting the future Chinese Large Optical/infrared Telescope(LOT)project.The measurements were obtained with differential image motion monitors(DIMMs)from April2017 to November 2018 at different heights during different periods.The median seeings at 11 m and6 m are very close but significantly different from that on the ground.We mainly analyzed the seeing at11 m monthly and hourly,having found that the best season for observing was from late autumn to early winter and seeing tended to improve during the night only in autumn.The analysis of the dependence on temperature inversion,wind speed and direction also was made and the best meteorological conditions for seeing are given.

Data processing and data products from 2017 to 2019 campaign of astronomical site testing at Ali,Daocheng and Muztagh-ata

Based on previous site testing and satellite cloud data,Ali,Daocheng and Muztagh-ata have been selected as candidate sites for the Large Optical/Infrared Telescope(LOT) in China.We present the data collection,processing,management and quality analysis for our site testing based on using similar hardware.We analyze meteorological data,seeing,background light,cloud and precipitable water vapor data from 2017 March 10 to 2019 March 10.We also investigated the relative usefulness of our all-sky camera data in comparison to that from the meteorological TERRA satellite data based on a night-by-night comparison of the correlation and consistency between them.We find a 6% discrepancy arising from a wide range of factors.