<p>A few things:</p>
<ul>
<li>The conversion between magnitudes is dependent on the filters being used, you can see it on the tables from the website you linked. That being said, the conversions available in there will not be useful for you if you are trying to replicate <a href="https://ui.adsabs.harvard.edu/abs/2005ApJ...631..163S/abstract" rel="nofollow noreferrer">Stern et al 2005</a>, as they use <em>redder</em> filters (in the website with the conversions, the longest wavelength available is 2.19 microns, whereas the shortest in Stern et al 2005 is 3.6 microns). If you can find the proper relationships, you should get the expected results.</li>
<li>The paper states that they used IRAC data, so it is a good idea to start looking over there. In fact, you can find some relations in point 7 <a href="https://irsa.ipac.caltech.edu/data/COSMOS/gator_docs/scosmos_irac_colDescriptions.html" rel="nofollow noreferrer">here</a> and you could also take a look at point 3, might be necessary (I do not know what data you are working with). If you are not using IRAC data, you might as well look for a different source for the conversions.</li>
<li>Are you perhaps using the <a href="https://www.strw.leidenuniv.nl/galaxyevolution/ULTRAVISTA/Ultravista/UltraVISTA_Catalog_Home.html" rel="nofollow noreferrer">UltraVISTA</a> catalog? If not, it might also be a good idea to check it out. Their <a href="https://www.strw.leidenuniv.nl/galaxyevolution/ULTRAVISTA/README_UVISTA_v4.1.txt" rel="nofollow noreferrer">README</a> contains some details about the photometry and references about how they acquired the data (including IRAC), which could give you some more hints.</li>
</ul>

CC BY-SA 4.0
2021-03-10T15:28:12.640Z
Looks much better now, thank you! It was indeed IRAC data
theWrongAlice
CC BY-SA 4.0
2021-03-08T00:08:24.890Z
2021-03-08T00:08:24.890Z
Solfreludio
<p>This might be a bit silly, but I'm really struggling with a conversion.
I have apparent <a href="https://en.wikipedia.org/wiki/AB_magnitude" rel="nofollow noreferrer">AB magnitudes</a>: <span class="math-container">$$m_{AB} = -2.5 \log_{10}(f_\nu [\mu Jy]) + 25$$</span> that I would like to convert to Vega magnitudes, because I am trying to make a figure similar to the one below from Stern et al. 2005. I used the conversions from this site: <a href="http://www.astronomy.ohio-state.edu/%7Emartini/usefuldata.html" rel="nofollow noreferrer">http://www.astronomy.ohio-state.edu/~martini/usefuldata.html</a>, but it came out all wrong. Should I not use these? Because the axes contain colors, I suppose I don't have to worry about the units of <span class="math-container">$f_\nu$</span>, because this will create an offset that cancels when you subtract two magnitudes, right? But I find that one color-conversion must be positive, because
<span class="math-container">$$[3.6]_{AB} - [4.5]_{AB} = [3.6]_{Vega} + 0.79 - [4.5]_{Vega} - (-0.09) \\= [3.6]_{Vega} - [4.5]_{Vega} + 0.88$$</span>
while the other is negative, because
<span class="math-container">$$[5.8]_{AB} - [8.0]_{AB} = [5.8]_{Vega} +0.02 - [8.0]_{Vega} - 0.45\\
= [5.8]_{Vega} - [8.0]_{Vega} - 0.43$$</span>
but if you look at this figure below, you see that you need two corrections with the same sign. Any thoughts on my attempt at conversion? Thank you.</p>
<p><a href="https://i.sstatic.net/oRIc1m.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oRIc1m.png" alt="Stern2005" /></a></p>

<photometry><apparent-magnitude><infrared>
How to convert between AB and Vega magnitudes?
theWrongAlice
# How to convert between AB and Vega magnitudes?

(Asked by: username_0 on 2021-02-15)

This might be a bit silly, but I'm really struggling with a conversion.
I have apparent [AB magnitudes](https://en.wikipedia.org/wiki/AB_magnitude): $$m\_{AB} = -2.5 \log\_{10}(f\_\nu [\mu Jy]) + 25$$ that I would like to convert to Vega magnitudes, because I am trying to make a figure similar to the one below from Stern et al. 2005. I used the conversions from this site: [http://www.astronomy.ohio-state.edu/~martini/usefuldata.html](http://www.astronomy.ohio-state.edu/%7Emartini/usefuldata.html), but it came out all wrong. Should I not use these? Because the axes contain colors, I suppose I don't have to worry about the units of $f\_\nu$, because this will create an offset that cancels when you subtract two magnitudes, right? But I find that one color-conversion must be positive, because
$$[3.6]\_{AB} - [4.5]\_{AB} = [3.6]\_{Vega} + 0.79 - [4.5]\_{Vega} - (-0.09) \\= [3.6]\_{Vega} - [4.5]\_{Vega} + 0.88$$
while the other is negative, because
$$[5.8]\_{AB} - [8.0]\_{AB} = [5.8]\_{Vega} +0.02 - [8.0]\_{Vega} - 0.45\\
= [5.8]\_{Vega} - [8.0]\_{Vega} - 0.43$$
but if you look at this figure below, you see that you need two corrections with the same sign. Any thoughts on my attempt at conversion? Thank you.

[![Stern2005](https://i.sstatic.net/oRIc1m.png)](https://i.sstatic.net/oRIc1m.png)

**Tags:** photometry, apparent-magnitude, infrared

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## 1 answers

### Answer by username_1 on 2021-03-08. Score: 2 | Accepted answer

A few things:

* The conversion between magnitudes is dependent on the filters being used, you can see it on the tables from the website you linked. That being said, the conversions available in there will not be useful for you if you are trying to replicate [Stern et al 2005](https://ui.adsabs.harvard.edu/abs/2005ApJ...631..163S/abstract), as they use *redder* filters (in the website with the conversions, the longest wavelength available is 2.19 microns, whereas the shortest in Stern et al 2005 is 3.6 microns). If you can find the proper relationships, you should get the expected results.
* The paper states that they used IRAC data, so it is a good idea to start looking over there. In fact, you can find some relations in point 7 [here](https://irsa.ipac.caltech.edu/data/COSMOS/gator_docs/scosmos_irac_colDescriptions.html) and you could also take a look at point 3, might be necessary (I do not know what data you are working with). If you are not using IRAC data, you might as well look for a different source for the conversions.
* Are you perhaps using the [UltraVISTA](https://www.strw.leidenuniv.nl/galaxyevolution/ULTRAVISTA/Ultravista/UltraVISTA_Catalog_Home.html) catalog? If not, it might also be a good idea to check it out. Their [README](https://www.strw.leidenuniv.nl/galaxyevolution/ULTRAVISTA/README_UVISTA_v4.1.txt) contains some details about the photometry and references about how they acquired the data (including IRAC), which could give you some more hints.

> **Comments:**
> **username_0**: Looks much better now, thank you! It was indeed IRAC data  

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