Selections on COMBO Catalogues

The selection of object samples from COMBO-17 catalogues may be tailored to specific applications using the classification information and flags. Simple queries for stars, galaxies or quasars, should just follow a selection on the first three letters in the column MC_CLASS ("Sta*", "Gal*", "QSO*", or "WD*"). The only sample that might suffer from significant contamination is the gaalxy class, as it includes all the objects which where too faint to be classified at all. However, as far more than 90% of these objects are galaxies anyway, and we know hardly more about them than their R-band magnitude and position, it might be totally irrelevant, if you get some quite faint stars and QSOs in there or not.

The catalogue contains all objects found by SExtractor, but a cut-off limit has been imposed later, at 3.5 sigma for the MAG_BEST measurement in the CDFS field, and at 4 sigma (longer exposure) in A901 and S11. The SExtractor flags are recorded in the column FLAG_ALL, and have been complemented by our own warning flags. In general, flag values ranging from 0 to 7 are ok. Sometimes 512..519 means a problem, but we suggest to check objects individually if they are valuable, as often they are still correctly classified and their magnitudes are still ok. Flag values of 8..255 or 1024 and above are fatal - these objects should be ignored entirely! Here are the meanings of the individual 'bits': 

   0   =  No problemo!
   1   =  Object has neighbors potentially biasing the SExtractor magnitude
   2   =  Object deblended by SExtractor
   4   =  Object contains (near-) saturated pixels
   8   =  Object truncated at image border  (fatal!)
   .                                        (fatal!)
  ..                                        (fatal!)
 128   =  Memory overflow during extraction (fatal!)
 512   =  MPIAPHOT signals a problem        (check object if valuable)
1024   =  unreal objscts, data artifacts    (fatal!)


Back to COMBO-17 index page

Last update Jun 23, 2003, CW