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I am an astronomy teacher, and made some kind of spectrograph with a difraction grating, a 3D printed slit, water pipes and a reflex camera. With a group of students we got this picture of the solar spectrum:

Solar spectrum

Then we created a luminosity profile with this program to study the position of the absorption lines, and we noticed a strange "bump" in the yellow part of the spectrum (I mark it below with a red curve):

Luminosity profile

We don't have a clue to what causes it. May be the camera? It's a Canon EOS T5i. We took the picture in RAW mode, and "debayered" it with this program: Fitswork.

Edit:

The diffraction grating is the Star Spectroscope, manufactured by Rainbow Optics. It's of the transmission type and, according to this site, it has 200 lines per milimeter.

Thank you very much in advance!

Edit 2:

As can be seen, the spectrograph is very... DIY. There are no other optical elements: just the slit, the grating, and the camera with it's zoom lens. Also, the grating is iluminated normally. enter image description here enter image description here enter image description here

The pictures through the camera look like this: enter image description here

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    $\begingroup$ Hmm so your asking about how the graph isnt fully accurate (I mean there is a lot of tiny bumps everywhere) and why basically? Because nothing is 100% accurate I guess $\endgroup$
    – DialFrost
    Oct 29, 2022 at 2:14
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    $\begingroup$ In fact, the tiny bumps are ok for me; I know that no observation is 100% accurate. The one which bothers me is the yellow one: I think it's well above the overall uncertainity of the rest of the profile. $\endgroup$ Oct 29, 2022 at 10:36
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    $\begingroup$ My guess is that this is from the grating itself. Unlike prisms they can have a lot of spectral structure. the normal procedure is to take a 2nd spectrum of a simple tungsten filament light source (a fancy light bulb) and use it to "calibrate" pixel by pixel another spectrum taken by the same apparatus. Can you tell us everything you know about the grating? What type (transmission or reflection), what material, groves per inch or cm, blaze angle, optimized wavelength range, etc.? Is it holographic or ruled, replica, etc.? Ideally a part number and manufacturer. Thanks! $\endgroup$
    – uhoh
    Oct 29, 2022 at 11:08
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    $\begingroup$ For example here's a similar yellow bump i.stack.imgur.com/uFyLW.gif from a Thorlabs transmission grating $\endgroup$
    – uhoh
    Oct 29, 2022 at 11:11
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    $\begingroup$ Thank you, uhoh! Now I'm at home; as soon I get to college next week I'll check the data about the grating and add it to the original post. But looking at that picture you sent, it seems to be that the cause. $\endgroup$ Oct 29, 2022 at 12:16

2 Answers 2

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The anomaly you have seen looks very much like a Rayleigh-Woods anomaly (sorry, I cannot find a good introductory reference but you could try Hessel & Oliner 1965). These tend to occur at wavelengths/diffraction angles where the second order diffracted wave is parallel to the surface of the grating and can either lead to dips or peaks in the grating efficiency that are much narrower than the expected diffraction grating response curve. It is difficult to confirm without you giving the full spectrograph setup, including details of the blaze angle if any and whether the spectrograph is being illuminated normally or at some other angle.

Rayleigh-Woods anomalies predominantly affect s-polarised light - conventionally, light that is polarised with the electric field perpendicular to the diffraction grating grooves. This might explain why you see different amplitudes for the effect when measuring the spectrum of various sources which might have different degrees of polarisation. A simple diagnostic would be to measure the spectrum of a polarised white-light source reflected off a mirror at its Brewster angle and in the appropriate orientation, and compare that with the spectrum of the unreflected, unpolarised source.

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  • $\begingroup$ I'm editing the question with more information... thank you! $\endgroup$ Dec 5, 2022 at 19:14
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(Actually, I'm not sure this is the real answer; I can revoke it as soon as someone shows me it's incorrect).

As suggested by @uhoh, we took a spectra of an halogen lamp, and as we also got an old incandescent lamp, we took one of it too:

enter image description here

As you can see, both lamps show a "bump" on the yellow part of the spectrum (more the tungten -old- lamp than the halogen). That seems to indicate that the diffraction grating is responsible for the "bump".

But, just to double-check, we also took two more spectra of the sun, in different conditions:

enter image description here

This made me realized that I didn't fully stated the conditions for taking the original spectra: as we weren't comfortable pointing the camera directly to the sun, in the first spectra we directed it to a cloud (it was a cloudy day). On another day, we took two more spectra: one pointing to the blue sky (it was a clear day), and another to a white painted screen reflecting the sun's rays. As you can see, the yellow "bump" is more prominent in the spectra taken pointing to the cloud!

So we thought that may be the water vapor has something to do with it, besides the diffraction grating.

Doing a fast internet search, we came across this paper: Operational parameters of thermal water vapor plasma torch and diagnostics of generated plasma jet, with this figure:

enter image description here

But the authors attribute the observed yellow bump to "noise". So it does not seem to explain our bump. :)

So, up to now, our preliminary (and certainly incomplete) conclusion to this topic question is that the yellow bump is due to our diffraction grating and the clouds we were pointing to that day.

Let me credit the other members of the team: Facundo Balerdi, Fabián Faingerch, Sofía Habu, Mariano Solís and Clara Telesca.

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