<p>I am trying to measure equivalent widths (EW) of absorption lines using spectra of a star, but I don't know if I am doing it correctly and would like to know your comments on my process (recommendations of software will also be appreciated). From the simplest definition, the area of the spectral line profile is A = I * W, where I is the continuum intensity (when constant, I assume) and W is the equivalent width, so W = A/I.</p>
<p>My spectrum is not normalized (I tried using iSpec but it didn't give me a good fit or any at all), so I was recommended do the EW measurement manually. To do this, I printed sections of my spectrum on millimeter paper, defined a local continuum and got the area “inside” the line by counting the squares. Then, I multiplied by the wavelength and flux conversion factors (say, how many units of wavelength were in 1 mm and the same for flux), and finally divided by the midpoint of the local continuum (I like to think of it as normalization). Is this correct?</p>
<p>Since I have to measure more than a hundred lines, I thought it was better to automate the process and I took this same principle to a code, getting the areas of rectangles that fit inside the line and dividing by the midpoint of the local continuum. However, when comparing the results between millimeter paper and code, the EW of a line varied by a factor of 5 (e.g., one line gave me W = 0.0543 nm using the first method, and the second 0.2885 nm). This brings me to the second question: how can the uncertainties of these measurements be calculated? I had seen in various media that one option is MCMC (for the code at least), but I'm not sure if I can apply it in my case.</p>
<p>I hope I'm making any sense and I'm sorry it's such a long text. Thanks for reading.</p>

CC BY-SA 4.0
2024-09-23T22:30:42.393Z
To use MCMC you need a model. You haven't described any model. I have found the software "splat" quite good for manual EW measurements.
ProfRob
CC BY-SA 4.0
2024-09-24T13:47:45.663Z
https://astro.dur.ac.uk/~pdraper/splat/splat.html
ProfRob
CC BY-SA 4.0
2024-09-24T16:09:09.763Z
Thank you so much! I'll give it a try.
Tali
<spectroscopy>
How to measure equivalent widths and its uncertainties?
Tali
# How to measure equivalent widths and its uncertainties?

(Asked by: username_0 on 2024-09-23)

I am trying to measure equivalent widths (EW) of absorption lines using spectra of a star, but I don't know if I am doing it correctly and would like to know your comments on my process (recommendations of software will also be appreciated). From the simplest definition, the area of the spectral line profile is A = I \* W, where I is the continuum intensity (when constant, I assume) and W is the equivalent width, so W = A/I.

My spectrum is not normalized (I tried using iSpec but it didn't give me a good fit or any at all), so I was recommended do the EW measurement manually. To do this, I printed sections of my spectrum on millimeter paper, defined a local continuum and got the area “inside” the line by counting the squares. Then, I multiplied by the wavelength and flux conversion factors (say, how many units of wavelength were in 1 mm and the same for flux), and finally divided by the midpoint of the local continuum (I like to think of it as normalization). Is this correct?

Since I have to measure more than a hundred lines, I thought it was better to automate the process and I took this same principle to a code, getting the areas of rectangles that fit inside the line and dividing by the midpoint of the local continuum. However, when comparing the results between millimeter paper and code, the EW of a line varied by a factor of 5 (e.g., one line gave me W = 0.0543 nm using the first method, and the second 0.2885 nm). This brings me to the second question: how can the uncertainties of these measurements be calculated? I had seen in various media that one option is MCMC (for the code at least), but I'm not sure if I can apply it in my case.

I hope I'm making any sense and I'm sorry it's such a long text. Thanks for reading.

**Tags:** spectroscopy

> **Comments:**
> **username_1**: To use MCMC you need a model. You haven't described any model. I have found the software "splat" quite good for manual EW measurements.  
> **username_1**: https://astro.dur.ac.uk/~pdraper/splat/splat.html  
> **username_0**: Thank you so much! I'll give it a try.  

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## 0 answers

