Running SExtractor for Source Detection and Deblending Þ SExtractor is used as a structural preprocessor to detect objects, create a segmentation image and determine the following five quantities for each object: X and Y coordinates (X_IMAGE, Y_IMAGE) Local sky background (BACKGROUND) Local sky sigma (BKGSIG) Isophotal area (ISOAREA_IMAGE) Þ SExtractor command: sex image.fits -c image.sex Make sure "CHECKIMAGE" is set to "SEGMENTATION" in image.sex Þ Parameter DEBLEND_MINCONT is particularly important: it specifies the fraction of the total object flux a "branch" must have in order for this branch to be considered as a separate object. The lower this parameter, the more eager SExtractor will be to separate a given object into multiple sub-components. The value is SUBJECTIVE. This parameter is your definition of an object. Þ Extract X_IMAGE, Y_IMAGE, BACKGROUND, BKGSIG, ISOAREA_IMAGE from the SExtractor output catalog file (example: image.cat) with the command: sexreduce -i image.cat -l X_IMAGE Y_IMAGE BACKGROUND BKGSIG ISOAREA_IMAGE -a The "-a" flag is necessary to produce an ASCII output. The output should be piped to a file with the extension ".gfxt", and this .gfxt should be used with GSCRIPTER (next) GIM2D Logfiles Þ Every iteration is stored! Þ Two lines are stored per iteration: line 1 shows parameter values and line 2 shows Metropolis temperatures Þ Each line starts with a letter code: F - ICF iteration I - Best ICF model R - Iteration rejected by Metropolis algorithm A - Iteration accepted by Metropolis algorithm before convergence E - Iteration accepted by Metropolis algorithm after convergence C - Rows of final covariance matrix P - Best-fit parameter, 99% lower bound, 99% upper bound Half-Light Radius Þ Computed by integrating best-fit bulge+disk profile out to infinity Þ This is a semi-major axis radius NOT a geometric mean
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