Return to Home Page
(fwd) a little red curiousity The following message is from John Greaves. The image is placed at http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/others/USNOcolor/Ru_versus_R2.gif === This doesn't actually amount to anything, but I find it an interesting curiousity anyroad. Seeing as POSS I has a pretty quick cut off in the south, I'd decided to try and see if USNO B1.0 B2-R2 could be of use with regards star colour assessment for the southern hemisphere. I'd thought about it before, but decided it wasn't worthwhile considering the lack of same date stuff, but something came up to make me do it anyway. B2 and R2 are the most commonly available for southern stuff, and using -30 degree declination northern cutoff I had about 8,000 stars that matched from Brian Skiff's loneos.phot. Anyway, I also matched these ~8000 R2 values against "UCAC red" mags, if only for the sake of it, and the graph is attached with R2 as horizontal axis and UCAC1 instrumental red as vertical axis. The black line represents where a linear 1:1 fit would lie. As with R1 against CMC12 r', a 1 to 1 fit more or less applies, and the standard deviation on R2 - R_ucac was 0.45 (though a plot of R2 - R_ucac is a 'banana'), much the same as in the case of R1 against CMC12 r' and B1 against loneos.phot list B, but note that despite that, it isn't actually a linear fit (R1 against r' was linear, as was B1 versus B). Ideally, as R2 is from F plates, and R1 from E plates, I ought to do an R2 versus CMC12 r' check to ensure it isn't something to do with F plates, but I can't see that (anyway, although the plates are the same, R2 around CMC12 zone is a different survey to R2 south of -30). But I reckon the non-linearity is in the UCAC magnitudes. Granted they are instrumental and not a standard in any way, and although called "red", stretch a fair bit into the visual, but that seems no reason for such an effect. Anyway, as I say, just a curiousity, but it does make me think on the nature of the plethora of instrumental CCD mags out there, seeing as most seem to use the same family of kodak chips etc. I know the non- linearity isn't marked, but folk do tend to quote their mags to high precision with no error estimates attached. Cheers John
Return to the Powerful Daisaku
vsnet-adm@kusastro.kyoto-u.ac.jp
