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I sometimes hear about astronomers using an arc spectrum to calibrate observations. For example a "He-Ar arc spectrum". What is an "arc" in this context? I assume it's nothing got to do with angles (arcmin, arcsec, etc.).

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  • $\begingroup$ @ConradTurner, you could add some more details and make an answer out of your comment ! $\endgroup$ – Py-ser Dec 1 '15 at 14:53
  • $\begingroup$ @LocalFluff See Conrad's answer. The arc refers to a discharge lamp. $\endgroup$ – Rob Jeffries Dec 2 '15 at 8:24
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An arc spectrum is one produced by a discharge lamp where the discharge is through ionised gas, in the case of He-Ar a mixture of Helium and Argon, which produces a predictable line emission spectrum.

They are often used to provide a calibration spectrum for spectrometers.

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  • $\begingroup$ An electric arc, in air, between two iron electrodes, is also used as a source of reference emission lines... $\endgroup$ – DJohnM Dec 5 '15 at 5:56

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