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Paper: The Challenge of Photometric Alignment: Proper Flat Fielding
Volume: 376, Astronomical Data Analysis Software and Systems XVI
Page: 313
Authors: Riffeser, A.; Goessl, C.A.; Koppenhoefer, J.
Abstract: Photometric alignment is crucial for a precise comparison of imaging data obtained under varying conditions; there are several effects that make these data differ by multiplicative and additive terms: moon and sky light, atmospheric extinction, optics (e.g. filter, focus, field distortion), dust, reflections (concentration) and scattered light (telescope baffle). For the flat-field correction, twilight images and dome flats are used to correct for pixel-to-pixel variations and for the absorption of the instrument including all multiplicative effects. We present an algorithm to combine all flat-field calibration frames very efficiently. An inhomogeneous illumination for those flat-field calibration frames may lead to systematic zero-point offsets depending on the position on the CCD. We present a solution to correct science frames with an optimal flat-field calibration frame using a simple observational method to measure the true illumination. We also discuss the importance of distinguishing between spatially varying absorption in the instrument and field distortion resulting from a non-flat optical plane. We present our theoretical description and the practical implementation to correct for the additive sky and multiplicative photometric terms.
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