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From John Greaves: On 2 Aug 2003, Mike Simonsen wrote: > For now, it seems best to use Tycho 2 V to ~10.5, ASAS-3 to ~12.5 and USNO > A2.0 V for fainter magnitudes and colors. This is contrary to what has been > suggested to me by others, but I just can't reconcile UCAC magnitudes to > anything useful past 13.0. In the North at least, usage of USNO B1.0 R1 and B1 might be preferable as they are at least same night observations. Also, as far as I can tell, 'USNO A2.0 V' comes from a simple formula Taichi Kato originally assessed in terms of USNO A1.0 blue and red. The photometric calibration was redone for A2.0. Tycho was used for A1.0. If Tycho2 was used for A2.0 (I think it was) this transferrance of usage for the V conversion is another likely cause of scatter. Cheers John PS Brian Skiff noted: > I think is mainly because John's test data wasn't any good either > below 13.0 In the first instance I will probably do an r'_CMCT versus USNO B1.0 R1 plot (with the latter being an approximation of Rc), if only because I can maintain overlap of field then. This should at least give clues with regards r'_CMCT at the faint end, even if again only untested against scattered, but this time using a dataset that is most likely at its best between red mags 13 and 17 and with a faint limit a fair bit below the CMC13 limit. [I'm a bit bandwidthly challenged at the moment, so it'll have to wait] Here's a REDucac1 versus USNO B1.0 R2 plot to illustrate the point. http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/others/USNOcolor/Ru_versus_R2.gif I'll maybes have a look at the SGP stuff after that.
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