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Imag from Vmag



 
 
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  #1  
Old March 26th 06, 09:42 AM posted to sci.astro.ccd-imaging
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Default Imag from Vmag

Anybody know how to compute I magnitude from V magnitude in the
Hipparcos catalog?

Thanks!

  #2  
Old March 26th 06, 06:27 PM posted to sci.astro.ccd-imaging
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Default 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  
Old March 27th 06, 10:30 PM posted to sci.astro.ccd-imaging
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Default 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  
Old March 28th 06, 03:15 AM posted to sci.astro.ccd-imaging
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Default 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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