<p>Spectral wavelengths usually are defined as vacuum wavelength. The index of refraction of the medium influences the wavelength, and as such 'in vacuum' is both easier to define and measure: You'd need to define and specify composition, pressure and temperature in addition to the wavelength for each measurement in air - all of which is unnecessary if you measure in vacuum - or at least you make comparison easier, if you give your measurements corrected to vacuum.</p>

CC BY-SA 4.0
2021-07-26T05:33:41.367Z
While vacuum wavelength would be the best of course, wavelengths in ambient air, or corrected to some [standard atmosphere](https://en.m.wikipedia.org/wiki/International_Standard_Atmosphere) have appeared in optics contexts. Can you support your answer by linking to a few catalogs of wavelengths that astronomers use and show that they do in fact list vacuum wavelengths? *Thanks!*
uhoh
CC BY-SA 4.0
2021-07-26T07:45:05.107Z
Agree with @uhoh, it's unfortunately not uncommon to use the wavelength in air in the astronomical literature.
pela
CC BY-SA 4.0
2021-07-26T04:00:34.090Z
2021-07-26T04:48:47.290Z
2021-07-26T04:48:47.290Z
planetmaker
<p>In the SDSS page <a href="https://classic.sdss.org/dr7/products/spectra/vacwavelength.html" rel="nofollow noreferrer">https://classic.sdss.org/dr7/products/spectra/vacwavelength.html</a>, it is written</p>
<blockquote>
<p>Because the SDSS observes many quasars at rest-frame ultraviolet
wavelengths, the data are stored in vacuum wavelengths.</p>
</blockquote>
<p>Does UV wavelength have something to do with storing data in vacuum wavelengths?</p>
<p>Also, when looking for emission lines in the spectrum of extragalactic objects, such as supernovae, when should we use vacuum wavelength as the central wavelength of the line
and when the air wavelength?</p>

CC BY-SA 4.0
2021-07-25T18:20:56.293Z
Others will probably step in with more practical knowledge, but I observe that air is more or less opaque to UV light at many wavelengths, so talking about the "spectrum in air" may not make much sense?
James K
CC BY-SA 4.0
2021-08-13T22:39:01.760Z
I've just asked [Where do Astronomers usually get their wavelengths? Where do they turn to look up standard, accepted values for spectral features?](https://astronomy.stackexchange.com/q/45290/7982)
uhoh
<observational-astronomy><spectroscopy>
Vacuum and air wavelengths in spectroscopy
Lekha
# Vacuum and air wavelengths in spectroscopy

(Asked by: username_0 on 2021-07-25)

In the SDSS page <https://classic.sdss.org/dr7/products/spectra/vacwavelength.html>, it is written

> Because the SDSS observes many quasars at rest-frame ultraviolet
> wavelengths, the data are stored in vacuum wavelengths.

Does UV wavelength have something to do with storing data in vacuum wavelengths?

Also, when looking for emission lines in the spectrum of extragalactic objects, such as supernovae, when should we use vacuum wavelength as the central wavelength of the line
and when the air wavelength?

**Tags:** observational-astronomy, spectroscopy
> **Comments:**
> **username_1**: Others will probably step in with more practical knowledge, but I observe that air is more or less opaque to UV light at many wavelengths, so talking about the "spectrum in air" may not make much sense?  
> **username_2**: I've just asked [Where do Astronomers usually get their wavelengths? Where do they turn to look up standard, accepted values for spectral features?](https://astronomy.stackexchange.com/q/45290/7982)  

---

## 1 answers

### Answer by username_3 on 2021-07-26. Score: 1

Spectral wavelengths usually are defined as vacuum wavelength. The index of refraction of the medium influences the wavelength, and as such 'in vacuum' is both easier to define and measure: You'd need to define and specify composition, pressure and temperature in addition to the wavelength for each measurement in air - all of which is unnecessary if you measure in vacuum - or at least you make comparison easier, if you give your measurements corrected to vacuum.

> **Comments:**
> **username_2**: While vacuum wavelength would be the best of course, wavelengths in ambient air, or corrected to some [standard atmosphere](https://en.m.wikipedia.org/wiki/International_Standard_Atmosphere) have appeared in optics contexts. Can you support your answer by linking to a few catalogs of wavelengths that astronomers use and show that they do in fact list vacuum wavelengths? *Thanks!*  
> **username_4**: Agree with @uhoh, it's unfortunately not uncommon to use the wavelength in air in the astronomical literature.  

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