Calculating Magnitudes and Creating Color/Color and Color/Magnitude Diagrams
What you will find below is a summary of some of the information on the following two
CoolWiki pages http://coolwiki.ipac.caltech.edu/index.php/Units section on magnitudes and
http://coolwiki.ipac.caltech.edu/index.php/Color-Magnitude_and_Color-Color_plots on CC
Diagrams and CM Diagrams. I would encourage you to read these pages to get a more
complete explanation of these ideas.
In your Excel spreadsheets you have already calculated the corrected flux values in
microjanskys (μJy) for your target stars. Magnitudes are just ratios of the flux of the target star
to the flux of a standard star. In the equations below M will represent magnitudes and F will
represent fluxes.
F 
M 1  M 2  2.5  log  2 
 F1 
The reference star that we will use is Vega which has a V magnitude very close to zero.
Unfortunately, it has a lot of dust so it does not have an actual zero magnitude in infrared,
although the values we will use have been adjusted to account for this and we can assume the
Vega infrared magnitude is zero. With Vega as star 2 in the equation above MVega=0 and the
flux FVega depends on the IRAC or MIPS wavelength you are working with which can be found
in the table below. Vega magnitude equation
 FVega 

M  2.5  log 
 F 


Vega flux values for several wavelength 2MASS, WISE, Spitzer IRAC & MIPS and more can
be found at http://coolwiki.ipac.caltech.edu/index.php/Central_wavelengths_and_zero_points.
The Spitzer values are below. NOTE that the zero point values are listed in janskys and need
to be converted to microjanskys in the formula above.
Band
I1
I2
I3
I4
M1
M2
M3
Wavelength (μm)
3.6
4.5
5.8
8.0
24
70
160
Zero Point (Jy)
280.9
179.7
115.0
64.13
7.14
0.775
0.159
An example of the calculation for a target star with a IRAC1 flux F3.6 = 5,000 μJy is below:
6
 FVega 
  2.5  log  280.9  10 uJy   11.87
M  2.5  log 
 5000 uJy 
 F 




The notation for magnitudes uses square brackets so [3.6] = 11.87
reminder that there is no telecon this week. you should read the page on source matching here:
> http://coolwiki.ipac.caltech.edu/index.php/C-CWEL_Source_Matching_Work
> as well as review the proposal we submitted for any questions. the comments from reviewers
are due into me by friday, so i hope to have them turned around and out to all of you no later
than wednesday 3/27, and hopefully before even then.
>
> if you have time and inclination, you could invest the time you would have spent on a telecon
on wednesday either on the IASC telecon that will be held during our regular meeting time
(you need to preregister so that i can send you the links just prior to the telecon), or watch the
ds9 tutorial that was held in january, linked here:
> http://coolwiki.ipac.caltech.edu/index.php/NITARP_tutorials
> we will be using ds9 for what we will be doing, so in terms of future skills you'll need for
working with me, frankly the ds9 one (or for that matter the WISE archive one from Nov 2012)
will be more useful for you than the IASC one.
>