Monitoring night-time cloud cover at the Muztagh-ata site

For optical astronomical observatories, the cloud coverage during night-time directly determines the available observing time (AOT) of a telescope, the most basic parameter in astronomical observations.

Using the ground-based all-sky cloud camera to monitor cloud amount is the fundamental step for astronomical observation site selection. Since the beginning of site testing in January 2017, the ground-based cloud detection has been carried out at the Muztagh-ata site.

Xu Jing, a senior engineer from Xinjiang Astronomical Observatory of the Chinese Academy of Sciences, and his collaborators have processed and analyzed the night-time ground-based cloud detection data collected at the Muztagh-ata site from 2017 to 2021. The results showed that the site could provide 175 clear nights in a year, in which 169 nights are with at least four hours of continuous observing time.

The study was published in Research in Astronomy and Astrophysics.

Statistical results of processed images showed 65% annual available observing night-time, 66% observing night-time with AOT more than 50%, and 51% of clear nights with AOT more than 75%. The amount of continuous observing time more than six hours during the night accounted for 31.1% of the total night-time.

Moreover, the researchers found discrepancies between the CM SAF cLoud, Albedo and surface RAdiation (CLARA) data and ground-based statistical results in the months with average cloud top heights lower than 3,500m from the ground. This might be caused by the limitation of the capability of ground-based all-sky cameras to recognize high clouds.

According to the researchers, these results could be applied to a wide range of cloud coverage analyzes.