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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?

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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.

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  • $\begingroup$ While vacuum wavelength would be the best of course, wavelengths in ambient air, or corrected to some 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! $\endgroup$
    – uhoh
    Jul 26 at 5:33
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    $\begingroup$ Agree with @uhoh, it's unfortunately not uncommon to use the wavelength in air in the astronomical literature. $\endgroup$
    – pela
    Jul 26 at 7:45

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