A couple of weeks ago, I thought I found a good potential target for DSLR photometry, RS CNC. It has a decent sized amplitide, and the publications that I have been able to find on this star gives a B-V color of about 1.35, but I am finiding that maybe it isn't so good of a target for DSLR measurements.
I'm now starting to crunch through some measurements, and when I compare my calculated V magnitude with other measurements made at the same time, I noticed that my results differ by about 0.05. I usually don't worry too much if my results differ from other observers by about 0.02, but 0.05 is a bit larger than usual.
So I wanted to see if perhaps the color index I was taking from catalogs was wrong. To do that, I would measure the magnitudes of various stars in the same image field using the DSLR Blue and DSLR Green channels. I would then plot a graph of B-V[catalog] versus DSLR(blue)-DSLR(green). I'm attaching a spreadsheet of the comparison. It shows a nice leanear relationship for the color index, at least in the range of -0.1 to 1.6 for B-V.
After I calculated the color for RS CNC, I got a value of 2.13, significantly redder than 1.35. I find this a bit puzzling, because my measurements of other stars that have published colors as red as 1.5-1.6 did not show any such discrepancies. So I now wonder if RS CNC can get significantly redder than the catalogs indicate, especially when it is in the dimmer part of it's cycle?
I don't have hard data yet, but after trying some tests with Betelgeuse, I think with stars are redder than about 1.7 or so, I am getting enough IR light leaking through the green filter so that it makes the star appear brighter in the green channel. Which in turn causes my V calculations to be brighter then they actually are.
Let me know what you think,
-Mike Durkin (DMPA)