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.

Site testing campaign for the Large Optical Telescope at the Ali site

The Large Optical/infrared Telescope(LOT)is a ground-based 12 m diameter telescope which is proposed to be built in western China.The site selection for LOT in China began in 2016,and Ali was listed as one of the three candidate sites.Remote studies and local surveys have been carried out for more than 15 years in western China,and the results show that Ali is a promising site with comprehensive quality in terms of atmospheric and supporting conditions.An overview of the site testing campaign at the Ali site from 2016 to 2019 is presented.After the two years of data collection,the overall median seeing value is found to be 1.17 arcsec,the observable nights are 81.71%and the good observable nights are 71.76%.The weather conditions as follows,the median night temperature value is-5.18℃,the median night relative humidity value is 41.25%,the median night atmospheric pressure value is 540.92 hPa,the median night wind speed value is 7.41 m s-1 and the mainly wind direction is southwestern(SW).The median night sky background value is 22.07 magV.We also discuss the wind speed at different locations on-site,the possibility of light pollution and the effect of wind speed on differential image motion monitor(DIMM)seeing measurements.

Seeing measurements for the Guoshoujing Telescope (LAMOST) site with DIMM

We present seeing measurements of the Guoshoujing Telescope （formerly named the Large Sky Area Multi-Object Fiber Spectroscopic Telescope- LAMOST） site at Xinglong station during the period from 2007 March 12 to April 25. The measurements were carried out with the Differential Image Motion Monitor （DIMM）, and a total of 9259 data sets was obtained. The median seeing was measured to be 1.1″, with 25% being better than 0.8″and 75% better than 1.5″. The experiment shows that the DIMM exposure time has significant effects on the results of seeing measurements. An SBIG Polaris seeing monitor, which had been planned to be installed on the LAMOST site for long-term monitoring, was also employed during the DIMM observations. The results show that the SBIG seeing monitor is easily affected by gusty wind, resulting in larger seeing values. Considering the previous seeing measurements at Xinglong station over the last 15 yr, we conclude that an acceptable seeing condition at Xinglong station is around 1″-2″.

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.

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.