<p>(Actually, I'm not sure this is the real answer; I can revoke it as soon as someone shows me it's incorrect).</p>
<p>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:</p>
<p><a href="https://i.sstatic.net/HCNRZ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HCNRZ.png" alt="enter image description here" /></a></p>
<p>As you can see, both lamps show a &quot;bump&quot; 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 &quot;bump&quot;.</p>
<p>But, just to double-check, we also took <em>two</em> more spectra of the sun, in different conditions:</p>
<p><a href="https://i.sstatic.net/xPqXk.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xPqXk.png" alt="enter image description here" /></a></p>
<p>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 <strong>cloud</strong> (it was a cloudy day). On another day, we took two more spectra: one pointing to the blue <strong>sky</strong> (it was a clear day), and another to a <strong>white painted screen</strong> reflecting the sun's rays. As you can see, the yellow &quot;bump&quot; is more prominent in the spectra taken pointing to the cloud!</p>
<p>So we thought that may be the water vapor has something to do with it, besides the diffraction grating.</p>
<p>Doing a fast internet search, we came across this paper: <a href="https://www.researchgate.net/publication/279183776_Operational_parameters_of_thermal_water_vapor_plasma_torch_and_diagnostics_of_generated_plasma_jet" rel="nofollow noreferrer">Operational parameters of thermal water vapor plasma torch and diagnostics of generated plasma jet</a>, with this figure:</p>
<p><a href="https://i.sstatic.net/T6Z4C.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/T6Z4C.jpg" alt="enter image description here" /></a></p>
<p>But the authors attribute the observed yellow bump to &quot;noise&quot;. So it does not seem to explain <em>our</em> bump. :)</p>
<p>So, up to now, our preliminary (and certainly incomplete) conclusion to this topic question is that <strong>the yellow bump is due to our diffraction grating <em>and</em> the clouds we were pointing to that day</strong>.</p>
<p>Let me credit the other members of the team: Facundo Balerdi, Fabián Faingerch, Sofía Habu, Mariano Solís and Clara Telesca.</p>

CC BY-SA 4.0
2022-11-29T18:47:26.250Z
2022-12-05T15:52:33.207Z
2022-12-05T15:52:33.207Z
Luis López
<p>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 <a href="https://opg.optica.org/ao/fulltext.cfm?uri=ao-4-10-1275&amp;id=13932" rel="noreferrer">Hessel &amp; Oliner 1965</a>). 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.</p>
<p>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.</p>

CC BY-SA 4.0
2022-12-05T19:14:08.483Z
I'm editing the question with more information... thank you!
Luis López
CC BY-SA 4.0
2022-12-05T17:11:27.637Z
2022-12-05T17:11:27.637Z
ProfRob
<p>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:</p>
<p><a href="https://i.sstatic.net/jUimX.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jUimX.jpg" alt="Solar spectrum" /></a></p>
<p>Then we created a luminosity profile with <a href="https://lopezsolerluis.github.io/annie-web/publicado/#" rel="nofollow noreferrer">this program</a> to study the position of the absorption lines, and we noticed a strange &quot;bump&quot; in the yellow part of the spectrum (I mark it below with a red curve):</p>
<p><a href="https://i.sstatic.net/jfGj2.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jfGj2.png" alt="Luminosity profile" /></a></p>
<p>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 &quot;debayered&quot; it with this program: <a href="https://www.fitswork.de/software/softw_en.php" rel="nofollow noreferrer">Fitswork</a>.</p>
<h2>Edit:</h2>
<p>The diffraction grating is the <strong>Star Spectroscope</strong>, manufactured by <em>Rainbow Optics</em>. It's of the transmission type and, according to <a href="https://www.rspec-astro.com/star-spectroscope/" rel="nofollow noreferrer">this site</a>, it has 200 lines per milimeter.</p>
<p>Thank you very much in advance!</p>
<h2>Edit 2:</h2>
<p>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.
<a href="https://i.sstatic.net/ZSh9u.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZSh9u.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/CA3Re.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CA3Re.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/8uStM.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8uStM.jpg" alt="enter image description here" /></a></p>
<p>The pictures through the camera look like this:
<a href="https://i.sstatic.net/qqrQy.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qqrQy.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2022-10-29T02:14:40.177Z
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
DialFrost
CC BY-SA 4.0
2022-10-29T10:36:40.600Z
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.
Luis López
CC BY-SA 4.0
2022-10-29T11:08:34.977Z
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!*
uhoh
CC BY-SA 4.0
2022-10-29T11:11:05.837Z
For example here's a similar yellow bump https://i.sstatic.net/uFyLW.gif 
from a [Thorlabs transmission grating](https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1123)
uhoh
CC BY-SA 4.0
2022-10-29T12:16:46.500Z
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.
Luis López
CC BY-SA 4.0
2022-10-29T21:46:59.460Z
@LuisLópez okay that's great! Once you find out you can post an answer to your own question, which is always okay in Stack Exchange. There are still plenty of tungsten light sources around even though the good-old incandescent screw-in light bulbs are hard to find. Tungsten halogen lights are at the DIY or hardware stores as replacement bulbs for some indoor lighting and for projectors, and for automobile + motorcycle headlights (though there are more and more LED or phosphor based competing technologies these days).
uhoh
CC BY-SA 4.0
2022-10-29T21:49:13.047Z
So I think you can take a spectrum of one of those for comparison. In the mean time why don't you ask a follow-up question about how astronomers calibrate the intensity scales of their spectrometers? We can guess how they calibrate the wavelength scale, but the intensity scale and its variation within the spectrum is also calibrated.
uhoh
CC BY-SA 4.0
2022-10-30T00:48:22.583Z
@uhoh '_why don't you ask a follow-up question about how astronomers calibrate the intensity scales of their spectrometers?_' Great idea! I'll post that question, and as soon I find out an answer, I'll post it here (if nobody else does it before... :)).
Luis López
CC BY-SA 4.0
2022-10-30T16:57:06.360Z
The last time I needed a tungsten lamp for calibration, I bought an “old school” MagLite flashlight, very inexpensively, on the well known auction site. The newer flashlights have LEDs, but the old type have tungsten bulbs with a little xenon in the bulb, if I remember correctly. Works great.
Ed V
CC BY-SA 4.0
2022-12-05T16:39:24.050Z
Please give the precise details of the spectrograph setup and the diffraction grating and collimator and camera that you are using.  The answer may well depend on these things. e.g. Is the diffraction grating blazed? Are you observing through a slit? Are there other optical elements, particularly mirrors that might have some polarisation selectivity? What is the wavelength sensitivity of your camera?
ProfRob
CC BY-SA 4.0
2022-12-05T16:44:48.553Z
Could there be any contamination from a sodium lamp anywhere?
ProfRob
CC BY-SA 4.0
2022-12-05T19:08:24.203Z
@ProfRob: I'm almost sure there wasn't; it was by day, and no street lamp was on. Besides, we were on a roof.
Luis López
<observational-astronomy><amateur-observing><photography><spectroscopy>
Strange bump in solar spectrum taken with home-made spectrograph
Luis López
# Strange bump in solar spectrum taken with home-made spectrograph

(Asked by: username_0 on 2022-10-29)

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](https://i.sstatic.net/jUimX.jpg)](https://i.sstatic.net/jUimX.jpg)

Then we created a luminosity profile with [this program](https://lopezsolerluis.github.io/annie-web/publicado/#) 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](https://i.sstatic.net/jfGj2.png)](https://i.sstatic.net/jfGj2.png)

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](https://www.fitswork.de/software/softw_en.php).

## Edit:

The diffraction grating is the **Star Spectroscope**, manufactured by *Rainbow Optics*. It's of the transmission type and, according to [this site](https://www.rspec-astro.com/star-spectroscope/), 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](https://i.sstatic.net/ZSh9u.jpg)](https://i.sstatic.net/ZSh9u.jpg)
[![enter image description here](https://i.sstatic.net/CA3Re.jpg)](https://i.sstatic.net/CA3Re.jpg)
[![enter image description here](https://i.sstatic.net/8uStM.jpg)](https://i.sstatic.net/8uStM.jpg)

The pictures through the camera look like this:
[![enter image description here](https://i.sstatic.net/qqrQy.png)](https://i.sstatic.net/qqrQy.png)

**Tags:** observational-astronomy, amateur-observing, photography, spectroscopy
> **Comments:**
> **username_1**: 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  
> **username_0**: 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.  
> **username_2**: 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!*  
> **username_2**: For example here's a similar yellow bump https://i.sstatic.net/uFyLW.gif  from a [Thorlabs transmission grating](https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1123)  
> **username_0**: 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.  
> **username_2**: @LuisLópez okay that's great! Once you find out you can post an answer to your own question, which is always okay in Stack Exchange. There are still plenty of tungsten light sources around even though the good-old incandescent screw-in light bulbs are hard to find. Tungsten halogen lights are at the DIY or hardware stores as replacement bulbs for some indoor lighting and for projectors, and for automobile + motorcycle headlights (though there are more and more LED or phosphor based competing technologies these days).  
> **username_2**: So I think you can take a spectrum of one of those for comparison. In the mean time why don't you ask a follow-up question about how astronomers calibrate the intensity scales of their spectrometers? We can guess how they calibrate the wavelength scale, but the intensity scale and its variation within the spectrum is also calibrated.  
> **username_0**: @uhoh '_why don't you ask a follow-up question about how astronomers calibrate the intensity scales of their spectrometers?_' Great idea! I'll post that question, and as soon I find out an answer, I'll post it here (if nobody else does it before... :)).  
> **username_3**: The last time I needed a tungsten lamp for calibration, I bought an “old school” MagLite flashlight, very inexpensively, on the well known auction site. The newer flashlights have LEDs, but the old type have tungsten bulbs with a little xenon in the bulb, if I remember correctly. Works great.  
> **username_4**: Please give the precise details of the spectrograph setup and the diffraction grating and collimator and camera that you are using.  The answer may well depend on these things. e.g. Is the diffraction grating blazed? Are you observing through a slit? Are there other optical elements, particularly mirrors that might have some polarisation selectivity? What is the wavelength sensitivity of your camera?  
> **username_4**: Could there be any contamination from a sodium lamp anywhere?  
> **username_0**: @ProfRob: I'm almost sure there wasn't; it was by day, and no street lamp was on. Besides, we were on a roof.  

---

## 2 answers

### Answer by username_4 on 2022-12-05. Score: 5 | Accepted answer

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](https://opg.optica.org/ao/fulltext.cfm?uri=ao-4-10-1275&id=13932)). 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.

> **Comments:**
> **username_0**: I'm editing the question with more information... thank you!  

---

### Answer by username_0 on 2022-11-29. Score: 4

(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](https://i.sstatic.net/HCNRZ.png)](https://i.sstatic.net/HCNRZ.png)

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](https://i.sstatic.net/xPqXk.png)](https://i.sstatic.net/xPqXk.png)

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](https://www.researchgate.net/publication/279183776_Operational_parameters_of_thermal_water_vapor_plasma_torch_and_diagnostics_of_generated_plasma_jet), with this figure:

[![enter image description here](https://i.sstatic.net/T6Z4C.jpg)](https://i.sstatic.net/T6Z4C.jpg)

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.


---

