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The following message is from John Greaves: Attached is usnobcmcr.gif, being a scatter plot of ~18500 objects between RA 0 and 90 degrees and Declination 0 and +1 degrees for USNO B1.0 R1 (a photographic red magnitude approximating Rc, but with scatter) and CMC13 r'_CMT (note I have been wrongly calling this r'_CMCT heretofore. CMT stands for Carlsberg Meridian Telescope). Although there is some vagary in the plot (it appears at least partway bimodal), and a not unexpected scatter, the fit appears linear enough to the faint end of about 16.5 here. Indeed, despite the look of the plot the correlation coefficient is a surprising 0.95 for the linear fit. Mean and median for R1 minus r'_CMT are 0.41 and 0.43, so not too skewed, with standard deviation a not unsurprising 0.31. Checking some documentation I note there is apparently an RA term with respect to CMC13 photometry, which although they've corrected for it, may have consequences in terms of scatter. Compare the attached plot with the following url showing REDucac1 versus USNO B1.0 R2 to get the feel of how the two compare. It seems the nonlinearity and larger REDucac2 - r'_CMT differences at the faint end are UCAC2 related. However, this still doesn't really answer the scatter issue, as USNO B1.0 R1 is well scattered itself. http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/others/USNOcolor/Ru_versus_R2.gif (granted this latter is an UCAC1 red versus USNO B1.0 southern R2 plot, ie F emulsion plates, as opposed to the UCAC2 red and R1 E emulsion plates of the attached gif) So, I still reckon you're better off using r'_CMT than REDucac2: the remaining question is whether you are in fact better off using neither!!! ;^) I dunno, people still frequently use USNO An.0 magnitudes as if they were in some way better than GSC ones, so there may be some application. [Heck, the ICQ recommends VT magnitudes for visual comet estimates, and down to 12.5ish, and I've never been able to get my brain around that!] Cheers John
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