Making plots was frustrating sometimes ... Marvin snickered a lot at our software because he's a Mac user and thought all this typing in at the command line was silly... but anyway here's how it went:
DAOphot gave us the magnitudes in B and V for all our stars, and a couple dreary details turned them into calibrated magnitudes, so next we were ready to make a CMD. We made a column of B-V values and then got plotting. Before long our first CMD (for M5) was ready for the world:
First thing we noticed: Surprise! Real astronomical data doesn't look like the diagrams in textbooks. But when we looked closely, it seemed like there were some features in our CMD that look like the places in textbooks. We definitely have a red-giant branch, and a horizontal branch, and probably the turn-off region. We looked at some CMDs from other clusters for comparison, like this one. We decided that we don't have a main sequence because we would have needed to take longer exposures to see stars that faint. (We really could have done that --- our exposures were only a minute or two long --- it's just that their were other COSMOS groups all observing that night too.)
But there's also a lot of junk in our CMD. Points way over on the blue side, way over on the red side, lots of scatter among the faint stars, etc. So we decided to take advantage of some more information we'd been ignoring.
DAOphot didn't just tell us the magnitudes of the stars; it also told us how uncertain it was about those magnitudes, and in general how well it was able to fit and "subtract" each star. So we cut out stars with very uncertain magnitudes (more than 0.1 magnitudes uncertain in B or V) and with not-star-like "sharp" values (further than 1 away from 0). Here's the CMD we got when we did that:
The result: now we are sure we have the top of the turn-off region, the red-giant branch, and the horizontal branch. Next we tried to use the CMD to determine the age and distance to M5 ...