J and K to BVRcIc: METHOD #0
SE, 2 Minutes
Bruce L. Gary, Hereford Arizona Observatory (G95)
The 2MASS (2 Micron All Sky Survey) star catalog contains over 1/2
million entries of good quality J, H and K magnitudes. It is ~99%
complete for stars having V-mag < 17.5. A typical amateur's FOV will
contain dozens of stars with good quality JHK magnitudes. Ever since
these magnitude bands were defined and measurements were made with them
it has been known that most stars have spectrae that are "well behaved"
in the sense that if you know a star's color with any two bands it is
possible to predict colors for any other band pair. Of course, the
strength of the color/color correlation depend on the star's surface
temperature (color), but with the advent of an immense data base of
quality JHK magnitudes it is possible to convert JK color to any of the
other color pairs with an accuracy that meets most needs. One of the
first comprehensive analyses that demonstrated the reliability of these
color/color relationships was published by Caldwell et al (1993).
Others followed, but my favorite, and the most recently published, is
by Warner and Harris (2007). They show that for the color region -0.1
< J-K < +1.0 it is possible to infer B, V, Rc and Ic magnitudes
with the following SEs: 0.08, 0.05, 0.04 and 0.035.
Last updated 2007.09.02
To calculate a star's magnitude for any of the standard visible bands (BVRcIc) do the following.
1) Determine the star's J and K magnitudes. This can be
done using TheSky/Six (select the UCAC2 information for the star).
If you don't have TheSky/Six, direct your web browser to http://irsa.ipac.caltech.edu/ and figure out how to get J and K from the IPAC catalog.
2) Convert from J-mag to BVRcIc using the following equations:
B = J + 0.198 + 5.215 * (J-K) - 2.7785 * (J-K)2 + 1.7495 * (J-K)3
V = J + 0.1496 + 3.5143 * (J-K) - 2.325 * (J-K)2 + 1.4688 * (J-K)3
Rc = J + 0.1045 + 2.5105 * (J-K) - 1.7849 * (J-K)2 + 1.123 * (J-K)3
Ic = J + 0.0724 + 1.2816 * (J-K) - 0.4866 * (J-K)2 + 0.2963 * (J-K)3
You can expect SE = 0.08, 0.05, 0.04 and 0.035 provided -0.1 < (J-K)< 1.0.
What could be easier!
You may want more accurate magnitudes, but any alternative for
achieving better accuracy will involve much more work. The only
exception is if you're dealing with a bright star having Tycho Bt and
Vt magnitudes, which can be converted to more accurate B and V
Caldwell JAR, Cousins AWJ, Ahlers CC et al (1993)
“Statistical Relationships Between the Photometric Colours of Common Types of
Stars in the UBVRcIc, JHK and uvby Systems,” South African Astronomical
Observatory Circular #15.
Warner, B. D. (2007) “Initial Results from a Dedicated H-G Project," Minor Planet Bulletin, 34-4, pages 113-119.
Return to calling web page: Photometry for Dummies
B and V to RcIc
If you only have B and V magnitudes you can estimate Rc and Ic using the following graphs (with equations included).
The caution for using tese two plots is that they are valid for main sequence stars, which constitute ~90% of stars.
B and V to RcIc
J and K magnitudes don't exist for bright stars, but those will surely
be included in the Tycho list. But Tycho's Bt and Vt have to be
adjusted to resemble B and V. Here's how this simple conversion works:
V = Vt - 0.090 * (Bt - Vt)
B = V + 0.820 * (Bt - Vt)
or, if you want more accuracy:
V = Vt + 0.00097 - 0.1334 * (Bt - Vt) + 0.05486 * (Bt - Vt)^2 - 0.01998 * (Bt - Vt)^3 (-0.25 < Bt - Vt < 2.0
B - V = (Bt - Vt) - 0.007813 * (Bt - Vt) - 0.1489 * (Bt - Vt)^2 + 0.03384 * (Bt - Vt)^3 (0.5 < Bt - Vt < 2.0
B - V = (Bt - Vt) - 0.006 - 0.1069 * (Bt - Vt) + 0.1459 * (Bt - Vt)^2 (-0.25 < Bt - Vt < 0.5
For more information about htis and other conversaion equations, click on this link: http://www.aerith.net/astro/color_conversion.html
This site opened: September
2007. Last Update: March