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周旭研究员
BATC课题组
中国科学院国家天文台,中国北京大屯路甲20号,邮编:100012
电话:86 10 64852571, E-mail: zhouxu@bao.ac.cn

English

Under Sampling Effect on CCD

and

its Modelling in BATC Photometric System

Intra-pixel effect in certain wide-field CCD imaging systems affects photometric precision profoundly. This is an under-sampling effect in essence and is caused by the variation of source position at a particular location on an individual pixel. To study this effect in detail, we monitored the bright stars in a 14x14 arc-min² field continuously in a whole night using BATC observing system. This 10-hour monitoring reveals a fact that all the stars in the field vary periodically with the same amplitude and the same frequency, but out of phase with one another. This variation diminished toward the end of night due to the worsening of seeing. This under-sampling effect can be modeled and removed if the intra-pixel positions of stars and seeing on the CCD chip are known.

Figure 1. The light curves of the 10 brightest stars in the first 5 hours of observation in the night of November 23rd. The axis labels are the same as in Figure 3, and the lines represent our model simulations based on intra-pixel position and seeing {\it see text}

Figure 2. The deviation of the observed magnitude versus vertical subpixel position (a), and versus horizontal position (b). The smoothed curve represents the variation due to the intra-pixel effect. The 20 bright stars were used to get this relation but only the data points of the brightest star were shown in the figure.

Figure 3. The magnitude variation versus the seeing in the image. The solid parabolic curve represents the amplitude in seeing variation. The 20 bright stars were used to get this relation but only the data points of the brightest star were shown in the figure.