<p>Note that they are related via the reddening law, i.e.
<span class="math-container">\begin{equation*}R_\lambda = \frac{A_\lambda}{E(B-V)}\end{equation*}</span>
where <span class="math-container">$A_\lambda$</span> is the extinction at wavelenght <span class="math-container">$\lambda$</span>, and <span class="math-container">$E(B-V)$</span> is the  color excess for <span class="math-container">$B-V$</span>. In literature is commonly used <span class="math-container">$E(B-V)$</span>, while in practice is usually needed <span class="math-container">$A_\lambda$</span>, to switch back and forth from apparent magnitude <span class="math-container">$m_\lambda$</span> to absolute magnitude <span class="math-container">$M_\lambda$</span> with
<span class="math-container">\begin{equation*}m_\lambda=M_\lambda+A_\lambda+(m-M)_0\end{equation*}</span>
using the absolute distance modulus <span class="math-container">$(m-M)_0$</span>.</p>

<p>Back to your question, classical references for the color excess of galactic globular clusters are the <a href="http://physwww.mcmaster.ca/~harris/mwgc.dat" rel="nofollow noreferrer">Harris catalog</a> and the <a href="https://irsa.ipac.caltech.edu/applications/DUST/" rel="nofollow noreferrer">Schlegel maps</a> (I would prefer the second). A nice solution is the Python module <a href="https://dustmaps.readthedocs.io/en/latest/index.html" rel="nofollow noreferrer">dustmaps</a> (<a href="https://joss.theoj.org/papers/10.21105/joss.00695" rel="nofollow noreferrer">Green 2018</a>); in the documentation you find all the available maps (<a href="https://dustmaps.readthedocs.io/en/latest/maps.html" rel="nofollow noreferrer">here</a>). </p>

<p>Finding <span class="math-container">$R_\lambda$</span> is more tricky. You can search in the literature for tabulated values (e.g. Tables 14 and 15 in <a href="https://ui.adsabs.harvard.edu/abs/2005PASP..117.1049S/abstract" rel="nofollow noreferrer">Sirianni et al. 2005</a>), otherwise a rough-but-still-working approach is to take a <span class="math-container">$R_\lambda$</span> vs. <span class="math-container">$\lambda$</span> plot (e.g. the one in <a href="https://ui.adsabs.harvard.edu/abs/1999PASP..111...63F/abstract" rel="nofollow noreferrer">Fitzpatrick 1999</a>) and to extrapolate by-hand <span class="math-container">$R_\lambda$</span> at the effective wavelength of your filter. Note that usually are presented several reddening laws, distinguished by <span class="math-container">$R_V$</span>. This is because they're not universal and depends on the environment; a reasonable choice for GGCs is <span class="math-container">$R_V=3.1$</span> .</p>

CC BY-SA 4.0
2020-01-04T21:21:19.983Z
2020-01-04T21:21:19.983Z
NGsp
<p>Where can I find a dataset containing the interstellar reddening and extinction values for all globular clusters, preferably in the HST/WFC3 system.</p>

<data-analysis><photometry><apparent-magnitude><extinction>
Extinction and reddening dataset
Hrsht
# Extinction and reddening dataset

(Asked by: username_0 on 2019-12-20)

Where can I find a dataset containing the interstellar reddening and extinction values for all globular clusters, preferably in the HST/WFC3 system.

**Tags:** data-analysis, photometry, apparent-magnitude, extinction

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## 1 answers

### Answer by username_1 on 2020-01-04. Score: 1 | Accepted answer

Note that they are related via the reddening law, i.e.
\begin{equation\*}R\_\lambda = \frac{A\_\lambda}{E(B-V)}\end{equation\*}
where $A\_\lambda$ is the extinction at wavelenght $\lambda$, and $E(B-V)$ is the color excess for $B-V$. In literature is commonly used $E(B-V)$, while in practice is usually needed $A\_\lambda$, to switch back and forth from apparent magnitude $m\_\lambda$ to absolute magnitude $M\_\lambda$ with
\begin{equation\*}m\_\lambda=M\_\lambda+A\_\lambda+(m-M)\_0\end{equation\*}
using the absolute distance modulus $(m-M)\_0$.

Back to your question, classical references for the color excess of galactic globular clusters are the [Harris catalog](http://physwww.mcmaster.ca/~harris/mwgc.dat) and the [Schlegel maps](https://irsa.ipac.caltech.edu/applications/DUST/) (I would prefer the second). A nice solution is the Python module [dustmaps](https://dustmaps.readthedocs.io/en/latest/index.html) ([Green 2018](https://joss.theoj.org/papers/10.21105/joss.00695)); in the documentation you find all the available maps ([here](https://dustmaps.readthedocs.io/en/latest/maps.html)).

Finding $R\_\lambda$ is more tricky. You can search in the literature for tabulated values (e.g. Tables 14 and 15 in [Sirianni et al. 2005](https://ui.adsabs.harvard.edu/abs/2005PASP..117.1049S/abstract)), otherwise a rough-but-still-working approach is to take a $R\_\lambda$ vs. $\lambda$ plot (e.g. the one in [Fitzpatrick 1999](https://ui.adsabs.harvard.edu/abs/1999PASP..111...63F/abstract)) and to extrapolate by-hand $R\_\lambda$ at the effective wavelength of your filter. Note that usually are presented several reddening laws, distinguished by $R\_V$. This is because they're not universal and depends on the environment; a reasonable choice for GGCs is $R\_V=3.1$ .


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