On the Exposure Time of the Mephisto-W Survey

The multi-channel photometric survey telescope (Mephisto) that is under construction, is expected to carry out the Mephisto-W Survey in 2022. The Mephisto-W survey is a multi-channel and multi-epoch photometric survey that covers the observable northern sky (about 26 000 sq. deg). The survey will provide an opportunity to describe the detailed structure of the Milky Way, to explore the formation and evolution history of galaxies, to accurately constrain the cosmology model, and to understand the nature of dark matter and dark energy. Based on the data from Astronomical Site Monitoring System (ASMS) in Lijiang Observatory during 2019, we have constructed models of the variations of values of seeing, cloud and sky brightness in Lijiang Observatory for a entire year by a machine learning method. Based on the models, we simply estimate the number of epochs and the expected limiting magnitudes of the Mephisto-W survey. To achieve the scientific goals of Mephisto-W survey, it is recommended to take the 2-time 20 s exposure mode. That is for each visit, we observe a given field twice, with each time of 20 s exposure time. In this mode, we are able to observe the survey about 3.8 times for both the ugi and vrz filter combinations. The expected 5 sigma r band depth of the single visit is 22.37 mag and co-added depth in one year 23.11 mag.