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Imag from Vmag
Anybody know how to compute I magnitude from V magnitude in the
Hipparcos catalog? Thanks! |
#2
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Imag from Vmag
I've tried to estimate I-band magnitudes for Tycho and Hipparchos
stars in the past. Here's one set of notes I made, for the Tycho-2 catalog. # This file contains information on a subset of stars from the Tycho-2 # catalog. The stars were selected to be good photometric references, # by satisfying all of these conditions: # # magnitude Bt 1.0 (discards incorrect "zero" magnitudes) # magnitude Vt 1.0 (discards incorrect "zero" magnitudes) # uncertainty in Bt 0.20 # uncertainty in Vt 0.20 # color -0.2 (Bt - Vt) 1.8 # no other Tycho-2 star within 20 arcsec # "solution" flag does not indicate double-star astrometric solution # # (This differs from an earlier subset in the uncertainty in magnitude: # 0.20 versus 0.05 mag. Almost all stars have larger uncertainty in Bt) # # For each such star, I estimated Johnson-Cousins magnitudes based on # on message from Arne Henden: # # B = Bt + 0.018 - 0.2580*(Bt - Vt) # V = Vt + 0.008 - 0.0988*(Bt - Vt) # I = Vt - 0.039 - 0.9376*(Bt - Vt) You may also find it worth while to look at http://stupendous.rit.edu/tass/catal...s.html#vi_type |
#4
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Imag from Vmag
SM, Chris,
Thank you for your help. Chris, could you clarify a couple of points: 1) "Between the errors introduced by the required transforms, and the quality of the data itself, I doubt you can expect better than a couple tenths of a magnitude in I in most cases" By "errors in transforms" do you mean that the formulas (similar to what SM suggests above) are statistical curves which result in rough approximations for any particular case? By "couple of tenths" do you mean that the error I mag should have an upper bound of 0.2? (Which is quite sufficient for my purposes, btw.) I am a newbie when it comes to stellar stuff (magnitudes and color temperature in particular), so it'll take me a bit to figure out exactly what you meant by "BT,VT - Johnson, then use VI". As far as "What data do you want to work with?"... I guess whatever's in the Hipparcos catalog. Thanks again. -Yegor Plam |
#5
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Imag from Vmag
Please do look at the graphs shown at the link
I mentioned in my earlier posting. You will see that there is quite a wide spread in the spectral types of stars with identical Tycho colors. That's partly due to errors in the Tycho magnitudes, and partly due to the intrinsic spread in properties of stars. If all you want is a bunch of I-band magnitudes for Tycho stars, you can grab them from a datafile the TASS group has been using for some years now. I picked a subset of stars from Tycho which have reasonably accurate photometry and no close Tycho companions. See http://spiff.rit.edu/tass/pipeline/p.../pipeline.html and read down a few pages to the point where the catalogs ("Big Version" and "Small Version") are described. I provide estimate I-band magnitudes along with estimate Johnson-Cousins B and V, based on the Tycho Bt and Vt. Michael Richmond |
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