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Up: A photometric catalogue

3. Detection of objects

Objects were automatically detected using the task DAOPHOT/DAOFIND. This task performs a convolution with a gaussian having characteristics previously chosen taking into account the seeing in each frame (FWHM of the star-like profiles in the image) as well as the CCD readout noise and gain. Then, objects are identified as the peaks of the convolved image that are higher than a given threshold above the local sky background (chosen as approximately equal to 5 tex2html_wrap_inline982 of the image mean sky flux). A list of detected objects is thus produced and interactively corrected on the displayed image so as to discard spurious objects, add undetected ones and dispose of false detections caused by the events flagged in the previous section (all of which concerning only a few percent of objects). Notice that all objects that were ``hand-added'' to the final list are both very faint and very low surface brightness ones, though still visible by eye inspection. The completeness of the catalogue is by no means dependent on this correction.

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