# Flux --> Apparent magnitude ( only 1 source )

1. Nov 8, 2014

### SpaceNerdz

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. Nov 12, 2014

### Bandersnatch

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