Sky Brightness Values in the SDSS g and r Bands at the Dome A of the Antarctica in 2009

For an astronomical observation site its sky brightness is among the most important factors for observation.In this paper we investigate the sky brightness values in the SDSS g and r bands at the Dome A on the Antarctica Plateau by using the data collected with the Chinese Small Telescope Array (CSTAR) during the winter of 2009.There are 251310 and 536383 image frames in the g and r bands, respectively.These frames correspond to total exposures of 969 hours and 2349 hours, respectively.In the data reduction we use two super-flat images combined from images taken at the Dome A, and use two bias images taken at the Xinglong Station of the National Astronomical Observatories of China.Variations of the sky brightness at the Dome A are mainly caused by changes in the solar elevation angle and the lunar phase.The CSTAR worked for a shorter time in the g band than in the r band.Nearly half of all images in the g band were taken when the sun was far below the horizon at the Dome A.The results show that in 74% of the g-band images and 91% of the r-band images sky brightness values are less than 100ADU/s.In less than 20% of the g-band images and less than 2% of the r-band images sky brightness values are over 300ADU/s.The median sky brightness values are 40ADU/s in the g-band and 28ADU/s in the r-band in the entire 2009 observational duty cycle of the CSTAR.Since most pixels in an image are in the sky region, the median values of the sky brightness values (in ADU per pixel) are evaluated using all the pixel values in the images.We use calibrated g- and r-band magnitudes of stars in the TYCHO catalog for photometric calibration.We have derived the zero points, g₀=3.76±0.07mag and r₀=3.94±0.06mag for the g and r bands, respectively, based on 12 of the brightest stars in the field of view of the CSTAR.Using the definition of the instrumental magnitudes in the CSTAR photometric system, a pixel size of 15 arcsec, and the zero points, we transform the sky brightness values in ADU/s into magnitudes.We do not consider the atmospheric extinctions since they are less than the errors of the zero points in clear photometric nights.We conclude that the median sky brightness values are 19.9mag/arcsec² in the g band and 20.1mag/arcsec² in the r band in the entire 2009 observational duty cycle of the CSTAR.