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\chapter{Measurements}
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Once sources have been detected and deblended, they enter the measurement phase.
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{\sc SExtractor} performs two categories of measurements. Measurements from the first
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category are made on the isophotal object profiles. Only pixels above
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the detection \index{threshold} threshold are considered. Many of these isophotal measurements
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(like {\tt X\_IMAGE}, {\tt Y\_IMAGE}, etc.) are necessary for the internal
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operations of {\sc SExtractor} and are therefore executed even if they are not
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requested. Measurements from the second category have access to all pixels of the \index{image} image.
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These measurements are generally more sophisticated and are done at a later stage
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of the processing (after {\tt CLEAN}ing and {\tt MASK}ing).
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