NYU Value-Added Galaxy Catalog
For each resolved SDSS object in the NYU-VAGC with a spectrum and within 1,500 km/s, or without a spectrum and with a half-light surface brightness brighter than 19, we have run an image analysis program to determine whether or not it is actually a double star. Basically, we fit a single PSF to the object and a double PSF (using the KL estimate of the PSF). If the double PSF is a much better fit than the single PSF, and not far worse than the SDSS model fit, than we classify the object as a double star. The settings are conservative in the sense that virtually all objects flagged as double stars are in fact double stars (or are otherwise poor deblends). Thus, the procedure does not flag ALL existing double star, only most of them. Most of the remaining we have caught in our eyeball quality checks.
The results are stored in:
$VAGC_REDUX/doublestar/doublestar.fitswhich has the following columns:
For the ISDOUBLE to be set, we require that MODELFLUX/PSFFLUX be less than 2, and that CHI2DOUBLE/CHI2SINGLE be less than 0.2. As noted above, this yields no false positives, but a number of false negatives. The user may use the results in the file to tweak these parameters, if they so desire (images for all checked objects are in the cutouts directory).