<p>On the Astrosurf website <a href="http://www.astrosurf.com/buil/echelle/first.htm" rel="noreferrer">here</a>, someone has designed a deceptively simple echelle spectrograph for viewing solar spectrum. Briefly, light from an optical fiber falls on an echelle grating and which is cross dispersed with a prism.</p>
<p><a href="https://i.sstatic.net/ZB3Fy.png" rel="noreferrer"><img src="https://i.sstatic.net/ZB3Fy.png" alt="amateur spectrograph" /></a></p>
<p>In theory, an echellogram has a range of wavelengths (free spectral range) in various orders as follows:</p>
<p><a href="https://i.sstatic.net/dr4fX.png" rel="noreferrer"><img src="https://i.sstatic.net/dr4fX.png" alt="echellogram" /></a></p>
<p>I tried to make a crude version of this (without an optical fiber or a collimating lens) for educational purposes.
<a href="https://i.sstatic.net/Lja5S.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Lja5S.jpg" alt="enter image description here" /></a></p>
<p>With a white LED (from an iphone or simple LED flashlight, or a LED bulb) shining on the echelle at a proper angle, one can generate a pattern as follows, which looks impressive and looks like a true echellogram. <strong>Unfortunately, this success is rather short lived. The moment we use a fluorescent lamp with a slit infront, we just get repeating images, which shows that this is not an proper echellogram!</strong> Does anyone know why this is happening? A senior spectroscopist has also tried it and no matter what orientation of slit or blaze direction (against or towards), grating angle, grating orientation... the result is just a repeating image.</p>
<p><strong>Questions</strong>:
a) Is it just because there is no collimator in our crude version? Why is an LED working and line source not? I tried collimation with a lens, but there was no improvement (repeating lines were still there).</p>
<p>b) If this were a proper echellogram of a line source, such as a fluorescent bulb, one should see one purple line in first row (higher orders), two green lines, two yellow lines (Hg doublets) and a few red lines. I cannot understand why there is a repetition of the same wavelength say purple line or the two green lines?</p>
<p>Thanks.</p>
<p><a href="https://i.sstatic.net/iaYSV.jpg" rel="noreferrer"><img src="https://i.sstatic.net/iaYSV.jpg" alt="enter image description here" /></a>
<a href="https://i.sstatic.net/NVGyJ.png" rel="noreferrer"><img src="https://i.sstatic.net/NVGyJ.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2020-09-03T07:14:14.377Z
The two images appear to have been taken with two quite different optical set ups. More details and diagrams are needed.
ProfRob
CC BY-SA 4.0
2020-09-03T09:37:04.367Z
Is that the grating with a blaze angle of 75°? Is it inclined such that the incident and diffracted beams have approximately the same difference from the blaze angle (i.e. 70° & 80° or 72° & 78°? The grating has to be used in the correct direction, did you find an arrow printed on the back indicating which direction it should face? The blazed surface is meant to be used from one side only.
uhoh
CC BY-SA 4.0
2020-09-03T12:33:33.277Z
@uhoh, I have tried almost all angles and inclinations and so has my senior spectroscopist friend (independently). It has been a couple of years of this "side" educational project so you can imagine all permutations and combinations of angles and orientations. This repeating image does not go away. I have a feeling that this is just a collimation issue that parallel rays are not  falling on the echelle. I don't have a collimator and a simple lens in not working.
ACR
CC BY-SA 4.0
2020-09-03T12:37:56.533Z
Rob, The optical of echelle+plus is the same. No matter what you do the repeating image is there with a line source. May be I should ask from Christian Buil who runs the astrosurf website. I have a feeling that LED works bc the light source is somewhat collimated and fluorescent lamp is not.
ACR
CC BY-SA 4.0
2020-09-03T12:49:54.557Z
Did you find an arrow as mentioned [here](https://chat.stackexchange.com/transcript/message/55205264#55205264)? Did you try from the opposite side? The "staircase" profile only works from one direction, as shown in images in [this answer](https://astronomy.stackexchange.com/a/38485/7982) and [here](https://i.sstatic.net/gsrwR.png) and [here](https://i.sstatic.net/R3Nsy.jpg). BTW you really must use a slit or a cheap multimode fiber bundle and probably you'll need to also collimate. Optics is challenging and it usually fails if things are done in a less-than-correct way.
uhoh
CC BY-SA 4.0
2020-09-03T13:11:33.337Z
@Uhoh, I have also tried a slit.  Yes, the grating has a pencil arrow. If you stand a night far away from a mercury lamp, you can see that if the angle of holding is right, the echelle grating becomes suddenly very bright with a beautiful spectrum. Holding a transmission grating crossed with it gives the same multiple images.When you are the "right" angle of incidence and diffraction, the diffraction pattern is markedly bright. If you use it in the "wrong" light and holding direction the image is not bright at all.
ACR
CC BY-SA 4.0
2020-09-03T13:12:15.740Z
@Uhoh, If you have an echelle, you can also try. Worth the demo. I think we need to try a point source and a lens in front to collimate it.
ACR
CC BY-SA 4.0
2020-09-05T19:39:51.970Z
@uhoh, It is indeed collimation issue. We tried collimation and a fiber optic, the multiple images vanish. Still a lot needs to be to get a better focus so as to reduce a specific line to a small spot rather than a big smear. LED light from iPhone works because it is quite collimated (I guess). Do you have any other suggestions?
ACR
CC BY-SA 4.0
2020-09-05T21:42:00.100Z
A fiber will provide the same function as a slit, but in both cases the outgoing light is diverging. A collimator would be a positive lens (or concave mirror) with the slit or fiber output at its focal point. What comes out of a collimator is a *parallel beam of light*. The word "collimation" means to "make parallel". https://i.sstatic.net/q5Y1G.gif You let the light coming from the slit or fiber (or fiber with a slit in front of it) expand over a distance of tens of centimeters until it is wide enough to fill a large area of the grating. Then you use a lens to make it parallel.
uhoh
CC BY-SA 4.0
2020-09-05T21:48:08.363Z
In spectrometers this is usually done with a concave mirror to avoid chromatic aberration, but you can use a lens for simple tests. Light from a fiber bundle will expand with an NA ([numerical aperture](https://en.wikipedia.org/wiki/Numerical_aperture)) of about 0.12 so at 20 cm it will have a half-width of about 2.5 cm or a full width of 5 cm. If you have a lens with f = 10 to 30 cm focal length, place it so that the fiber is at a distance f from the lens to make a parallel (collimated) beam. Maybe you can do this with a cardboard tube and tape, but it's not easy :-)
uhoh
CC BY-SA 4.0
2020-09-05T21:51:26.103Z
By the way you will never get the full resolution that the high-dispersion echelle grating can provide unless you have a narrow slit and *excellent* collimation. So don't expect to see hundreds of narrow absorption lines in sunlight until you build a solid collimation system and focus it well. You may be able to get the cross-dispersed lines of different colors quite easily, but you won't see the very fine spectral features within them until you have excellent focus.
uhoh
CC BY-SA 4.0
2020-09-05T21:51:34.027Z
@uhoh, I know how difficult collimation is. I used to demonstrate really good quality monochromators and the show was spectular. The lenses I have are not achromatic. Do you have a good suggestion what type of collimator one can buy from Edmund or Thor?
ACR
CC BY-SA 4.0
2020-09-05T21:57:32.810Z
Oh, Thorlabs has both "cage mount" and "tube mount" systems that work very nicely and come in different sizes! You can get fiber connectors and lens mounts that fit in either one https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=69 This would allow you to hold the lens and fiber at a fixed or an adjustable distance. If you can get a physical paper catalog and look through it (it's hundreds of pages) you can see a wide variety of different mounting systems.
uhoh
CC BY-SA 4.0
2020-09-14T11:37:28.990Z
Isn't your figure similar to the echrllogram you have posted? Shouldn't the lamp have a continuum as well?
Alchimista
<amateur-observing><spectroscopy><optics>
A simple echelle spectrograph for viewing solar spectrum
ACR
# A simple echelle spectrograph for viewing solar spectrum

(Asked by: username_0 on 2020-09-03)

On the Astrosurf website [here](http://www.astrosurf.com/buil/echelle/first.htm), someone has designed a deceptively simple echelle spectrograph for viewing solar spectrum. Briefly, light from an optical fiber falls on an echelle grating and which is cross dispersed with a prism.

[![amateur spectrograph](https://i.sstatic.net/ZB3Fy.png)](https://i.sstatic.net/ZB3Fy.png)

In theory, an echellogram has a range of wavelengths (free spectral range) in various orders as follows:

[![echellogram](https://i.sstatic.net/dr4fX.png)](https://i.sstatic.net/dr4fX.png)

I tried to make a crude version of this (without an optical fiber or a collimating lens) for educational purposes.
[![enter image description here](https://i.sstatic.net/Lja5S.jpg)](https://i.sstatic.net/Lja5S.jpg)

With a white LED (from an iphone or simple LED flashlight, or a LED bulb) shining on the echelle at a proper angle, one can generate a pattern as follows, which looks impressive and looks like a true echellogram. **Unfortunately, this success is rather short lived. The moment we use a fluorescent lamp with a slit infront, we just get repeating images, which shows that this is not an proper echellogram!** Does anyone know why this is happening? A senior spectroscopist has also tried it and no matter what orientation of slit or blaze direction (against or towards), grating angle, grating orientation... the result is just a repeating image.

**Questions**:
a) Is it just because there is no collimator in our crude version? Why is an LED working and line source not? I tried collimation with a lens, but there was no improvement (repeating lines were still there).

b) If this were a proper echellogram of a line source, such as a fluorescent bulb, one should see one purple line in first row (higher orders), two green lines, two yellow lines (Hg doublets) and a few red lines. I cannot understand why there is a repetition of the same wavelength say purple line or the two green lines?

Thanks.

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

**Tags:** amateur-observing, spectroscopy, optics

> **Comments:**
> **username_1**: The two images appear to have been taken with two quite different optical set ups. More details and diagrams are needed.  
> **username_2**: Is that the grating with a blaze angle of 75°? Is it inclined such that the incident and diffracted beams have approximately the same difference from the blaze angle (i.e. 70° & 80° or 72° & 78°? The grating has to be used in the correct direction, did you find an arrow printed on the back indicating which direction it should face? The blazed surface is meant to be used from one side only.  
> **username_0**: @username_2, I have tried almost all angles and inclinations and so has my senior spectroscopist friend (independently). It has been a couple of years of this "side" educational project so you can imagine all permutations and combinations of angles and orientations. This repeating image does not go away. I have a feeling that this is just a collimation issue that parallel rays are not  falling on the echelle. I don't have a collimator and a simple lens in not working.  
> **username_0**: Rob, The optical of echelle+plus is the same. No matter what you do the repeating image is there with a line source. May be I should ask from Christian Buil who runs the astrosurf website. I have a feeling that LED works bc the light source is somewhat collimated and fluorescent lamp is not.  
> **username_2**: Did you find an arrow as mentioned [here](https://chat.stackexchange.com/transcript/message/55205264#55205264)? Did you try from the opposite side? The "staircase" profile only works from one direction, as shown in images in [this answer](https://astronomy.stackexchange.com/a/38485/7982) and [here](https://i.sstatic.net/gsrwR.png) and [here](https://i.sstatic.net/R3Nsy.jpg). BTW you really must use a slit or a cheap multimode fiber bundle and probably you'll need to also collimate. Optics is challenging and it usually fails if things are done in a less-than-correct way.  
> **username_0**: @username_2, I have also tried a slit.  Yes, the grating has a pencil arrow. If you stand a night far away from a mercury lamp, you can see that if the angle of holding is right, the echelle grating becomes suddenly very bright with a beautiful spectrum. Holding a transmission grating crossed with it gives the same multiple images.When you are the "right" angle of incidence and diffraction, the diffraction pattern is markedly bright. If you use it in the "wrong" light and holding direction the image is not bright at all.  
> **username_0**: @username_2, If you have an echelle, you can also try. Worth the demo. I think we need to try a point source and a lens in front to collimate it.  
> **username_0**: @username_2, It is indeed collimation issue. We tried collimation and a fiber optic, the multiple images vanish. Still a lot needs to be to get a better focus so as to reduce a specific line to a small spot rather than a big smear. LED light from iPhone works because it is quite collimated (I guess). Do you have any other suggestions?  
> **username_2**: A fiber will provide the same function as a slit, but in both cases the outgoing light is diverging. A collimator would be a positive lens (or concave mirror) with the slit or fiber output at its focal point. What comes out of a collimator is a *parallel beam of light*. The word "collimation" means to "make parallel". https://i.sstatic.net/q5Y1G.gif You let the light coming from the slit or fiber (or fiber with a slit in front of it) expand over a distance of tens of centimeters until it is wide enough to fill a large area of the grating. Then you use a lens to make it parallel.  
> **username_2**: In spectrometers this is usually done with a concave mirror to avoid chromatic aberration, but you can use a lens for simple tests. Light from a fiber bundle will expand with an NA ([numerical aperture](https://en.wikipedia.org/wiki/Numerical_aperture)) of about 0.12 so at 20 cm it will have a half-width of about 2.5 cm or a full width of 5 cm. If you have a lens with f = 10 to 30 cm focal length, place it so that the fiber is at a distance f from the lens to make a parallel (collimated) beam. Maybe you can do this with a cardboard tube and tape, but it's not easy :-)  
> **username_2**: By the way you will never get the full resolution that the high-dispersion echelle grating can provide unless you have a narrow slit and *excellent* collimation. So don't expect to see hundreds of narrow absorption lines in sunlight until you build a solid collimation system and focus it well. You may be able to get the cross-dispersed lines of different colors quite easily, but you won't see the very fine spectral features within them until you have excellent focus.  
> **username_0**: @username_2, I know how difficult collimation is. I used to demonstrate really good quality monochromators and the show was spectular. The lenses I have are not achromatic. Do you have a good suggestion what type of collimator one can buy from Edmund or Thor?  
> **username_2**: Oh, Thorlabs has both "cage mount" and "tube mount" systems that work very nicely and come in different sizes! You can get fiber connectors and lens mounts that fit in either one https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=69 This would allow you to hold the lens and fiber at a fixed or an adjustable distance. If you can get a physical paper catalog and look through it (it's hundreds of pages) you can see a wide variety of different mounting systems.  
> **username_3**: Isn't your figure similar to the echrllogram you have posted? Shouldn't the lamp have a continuum as well?  

---

## 0 answers

