Modeling the CCD Undersampling Effect in the BATC Photometric System

Abstract

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 field using the Beijing-Arizona-Taipei-14 # 14 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.

Image Reconstruction Using Variable Exponential Function Regularization for Wide-Field Polarization Modulation Imaging

“…Nevertheless, as degradation imposed by the experimental setup, it is impossible to obtain a perfectly sharp image of the specimen. The specimen medium contains retardancy [3,4], and charge-coupled devices (CCDs) can lead to discrete under-sampling [5,6], while the coupled wavefronts consisting of the polarization state of the light and the anisotropic distribution of the specimen can result in vectorial phase fitting degradation [7,8]. Since the uncertain estimation of the degradation imaging system, it is very difficult to accurately determine the point-spread function (PSF).…”

Intrapixel effects of CCD and CMOS detectors

“…If we consider nonuniformity within each pixel, either because of the material or electric field inside, then the measured flux of an undersampled image also depends on the true position of the star's centroid within the pixel. Such an intrapixel effect is present not only in front-side illuminated (FSI) devices but, to a lesser degree, also in backside illuminated (BSI) devices [7][8][9][10]. However, as we show in this work, even in the case where sampling is reasonably good, e.g., with a radius encircling 80% energy (R EE80 ) of the point spread function (PSF) of 2 pixels, intrapixel effects may still be relevant to projects like the LSST.…”