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Fig. 1.


Example of a reduced CCD image frame with extracted sources highlighted with red ellipses, and the target and comparison stars in green and blue, respectively. The ellipses shown here do not represent the optimum apertures, but are only for illustrative purposes. The dashed lines around the target and comparison stars represent the annuli that can be used to calculate the sky background, whereas the yellow ellipse indicates the location of a “sky star” that can also be used to calculate and subtract the sky background.

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