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R Geminorum is a magnitude +6 to +11 variable star notable for the discovery of technetium spectral lines. As the longest lived isotope is only a few million years, this was direct evidence that nucleosynthesis happens in stars via the s-process.

How easy would one of those technetium absorption lines be to see by an amateur astronomer?

Could I get a high dispersion plastic diffraction grating (say 1200 lines/mm) from a science store, hold it between the eyepiece of a telescope and a cellphone camera and have a snowball's chance on R Geminorum of seeing a technetium line?

Or are they really weak and require a large aperture and long exposure and sophisticated spectrograph to detect?

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    $\begingroup$ @KavinIshwaran A modest size amateur telescope, one that it wouldn't look ridiculous to use a cell phone with. I don't want to pre-constrain the answer to a specific number, let's see how it goes. $\endgroup$
    – uhoh
    Commented Dec 23, 2021 at 7:48
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    $\begingroup$ I have analyzed, taking a 130mm telescope with 10 mm eyepiece you can see R Germimorum, But to my view, the star itself would appear as a faint red dot, and if you have to place a plastic diffraction grating and then a camera, the most probable image you see in the camera is a blank screen, since the camera wont be able to pick up such faint light (maybe I am not sure since I dont know exact specs of the equipments) and the diffraction grating may not be showing you precise spectral lines since the image you get is small and faint in light $\endgroup$
    – Kavin Ishwaran
    Commented Dec 23, 2021 at 8:23
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    $\begingroup$ How would you propose to identify the Technetium line(s) from all the other lines? $\endgroup$
    – ProfRob
    Commented Dec 23, 2021 at 8:32
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    $\begingroup$ @ProfRob I would post an Astronomy Stack Exchange question asking for guidance, how else? :-) $\endgroup$
    – uhoh
    Commented Dec 23, 2021 at 13:36
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    $\begingroup$ @EdV thanks! Maybe I will just post a short answer there that will tag along with the question once they figure out where to migrate it. $\endgroup$
    – uhoh
    Commented Jan 16, 2023 at 1:09

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No, even if you could resolve the spectral lines, there is no way in your proposed experiment that you could distinguish the Technetium features from spectral lines due to many other chemical elements.

You would need to (a) stabilise your primitive spectrograph extremely securely and (b) have a standard wavelength source (maybe a copper-neon arc lamp) that you could also insert into the beam in order to calibrate your wavelength scale.

Even then, I would expect you would need an exposure time of at least 10 minutes to get much with a 13cm telescope on a 6th magnitude star at a resolution (say better than 0.1 nm) required to resolve weak spectral features. There are too many imponderables to be more precise.

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  • $\begingroup$ Okay I will upgrade my gedankenexperimental equipment and lower my aspirations and look for an easier first amateur spectroscopy experiment. Thanks! $\endgroup$
    – uhoh
    Commented Dec 26, 2021 at 0:43

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