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Flux --> Apparent magnitude ( only 1 source )

  1. Nov 8, 2014 #1
    Hi guys,

    I've seen a lot of textbooks converting between 2 apparent magnitudes to the ratio of 2 fluxes. But I just want to know how to convert 1 ( ONE, UNO , 1, NOT 2 ) flux to 1 ( ONE, UNO, 1, NOT 2 ) 1 Apparent Magnitude. I think I saw my professor wrote this down, but am not sure . Is this correct :

    Apparent Magnitude (m) = -2.5 log (Flux)

    Again, I want to stress that I only want to know the magnitude/flux of 1 ( ONE, UNO , 1, NOT 2 ) source, and I don't want to / don't care / really don't care about 2 sources with different magnitudes/ have absolutely no information on any other reference magnitude from another source.
  2. jcsd
  3. Nov 12, 2014 #2


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    Gold Member

    Hi SpaceNerdz,

    Unfortunately, your request is in the same category as asking for a stick with just one end.

    That is to say, the scale of apparent magnitude is by definition a scale that uses a fixed reference flux for its zero point.
    To put it another way, magnitude X in EM band Y tells you how much weaker/stronger is the flux of the observed star when compared to a star with flux Z.

    The closest you can get to "eliminating" the reference from the equation is to use units of flux where the m=0 reference flux is equal to 1, and flux of the target star is expressed as a fraction of the reference.
    In such case the equation:

    ##m_x-m_0= -2.5 log (F_x/F_0)##

    reduces to

    ##m_x= -2.5 log (F_x)##

    since ##m_0=0## and ##F_0=1##

    So, say, if the target star had 10 times the flux of the reference in a given band, you'd get apparent magnitude:

    ##m_x= -2.5 log (10)=-2.5##

    Still that is just visually hiding the reference.

    This link has got a table of standard reference fluxes for various bands:
    http://www.astro.utoronto.ca/~patton/astro/mags.html [Broken]
    (the "photon flux" table half way down)
    Last edited by a moderator: May 7, 2017
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