For the last few days I've spent some time constructing Hertzsprung-Russell diagrams from images of globular clusters (and some open clusters) that I've acquired over the years...
http://www.princeton.edu/~rvdb/images/NJP/HRdiagMatlab.html I wrote a Matlab program (available from the link on the page above) to identify "all" of the stars in the image and compute their magnitude and color and then display an HR diagram. I thought it would be fun to compare my charts against charts made from catalog data. In CdC I can make an "object list" of all stars in the field of view. Using ucac3, there are several hundred stars in, say, M13. The ucac catalog lists magnitudes for these stars but gives 0.0 as the B-V color index of most of these stars (and ALL of the centrally positioned ones). I tried constructing an HR diagram from just these stars but it fails to show the characteristic structure one expects to see of a globular cluster. Perhaps I should be using a different catalog. Any ideas? --Bob
