identify round, extended objects in our catalog. We define sharpness to be the ratio of flux in the brightest pixel to the “total” flux — that measured within the SExtractor automatic Kron aperture. This is essentially a difference of aperture magnitudes that measures the concentration of the object. This approach benefits from a nicely oversampled PSF (5 pixels per FWHM). Because the point spread function varies across the field-of-view, we normalized both sharpness and FWHM to the local value. Thus, objects that are extended have normalized sharpnesses less than unity, and normalized FWHMs greater than unity.

Figure \[figure:sharpFWHM\] shows plots of normalized sharpness versus normalized FWHM for the CTR field. The bottom plot is a zoomed-in version of the top. The black dots are objects in the CTR field with $V<20.5$ and $U<23$. Most of the objects cluster around (1,1), as we would expect for a sample dominated by stars. However, there is a fan of points toward lower sharpness and higher FWHM. Overplotted in red are [*all*]{} spectroscopically confirmed GCs, and in blue are all spectroscopically confirmed foreground stars. This is not the training set we had to work with initially, but we plot the end-product catalog for the 