Modeling the CCD undersampling effect in the BATC photometric system

In certain CCD imaging systems, "intrapixel effects" introduce nonnegligible errors into the photometric accuracy of these systems. This effect is presumably caused by undersampling of the point source image by individual CCD pixels. This undersampling effect shows that the exact location of the source image on a pixel determines the flux detected by that pixel. Thus, as the image center drifts, for various reasons, across a pixel in a series of exposures, a difference in the response from the center to the edge of a pixel results in a variation of flux readout by that pixel. This effect could be important under superb seeing conditions, mimicking microvariability in stellar photometry for imaging systems having certain plate scales. To study this effect in detail, we carried out a full-night monitoring of bright stars in a 14′ × 14′ field using the Beijing-Arizona-Taipei-Connecticut (BATC) observing system. This 10 hr monitoring program shows that all the stars in the field vary periodically with almost the same amplitude and frequency, but that the variations were out of phase among the stars in the field. Furthermore, the variation of all the stars diminished toward the end of the night, as the seeing became worse. Since the flux variation correlates well with the precise location of the image center on a pixel, we attribute these phenomena to the "intrapixel effect" of our CCD imaging system. In this study, we describe the nature of this undersampling effect and its influence on the photometric accuracy. We found that this effect can be modeled and removed if the seeing conditions and the intrapixel positions and motions of stars on the CCD chip are known.