<p>The echelle grating spectrometer has two dispersing elements. The first one is a high-resolution echelle grating that disperses light onto a second disperser, a low-resolution cross disperser that disperses in the perpendicular direction. This cross disperser can be either a prism or an echelle grating. The different high orders from the high-res disperser overlap on the cross disperser, with each order naturally slightly offset from the previous one. This design ensures that only certain wavelengths of each order land on the second disperser. In the diagram, only the red light of order 70 falls on the second disperser, and only the blue light of order 160 falls on the disperser, etc. The CCD captures bands of light, each representing a wavelength range with some overlap with adjacent bands.</p>

CC BY-SA 4.0
2024-09-18T16:14:40.437Z
2024-09-18T16:14:40.437Z
eshaya
<p>I'm learning about diffraction gratings and came across this graphic of NEID's spectra: <a href="https://i.sstatic.net/LhpieeGd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/LhpieeGd.png" alt="spectral format of NEID" /></a>. It shows that for each order in the spectra there is a small set of wavelengths (e.g. order 70 has many shades of red but no greens or blues). I was wondering conceptually why each order is associated with one color as opposed to showing up as a rainbow/full spectrum. I know about the grating equation but I'm trying to think of this conceptually as opposed to using math.</p>

CC BY-SA 4.0
2024-09-17T19:16:56.110Z
CCD Stitch boundaries are a technical limitation of CCD sensors and require processing to deal with. Aside from that I know very little, but the source of this image might be helpful. It looks like it has two axes but there is no indication of a key etc. It just seems like an artist's conception of long and short wavelengths
Yorik
CC BY-SA 4.0
2024-09-17T20:34:58.037Z
This is the source of the image: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9908/1/Design-of-NEID-an-extreme-precision-Doppler-spectrograph-for-WIYN/10.1117/12.2234411.full#_=_ Though I do not think the axes are relevant as I can find another image that demonstrates the same question without axes (and without CCD stitch boundaries for that matter) (see image here https://astronomy.stackexchange.com/questions/38706/a-simple-echelle-spectrograph-for-viewing-solar-spectrum)
nittany_astro
CC BY-SA 4.0
2024-09-17T22:19:46.900Z
There is a range of wavelengths in each grating order of the echelle spectrum. See examples of real echelle spectra in my answer here: https://astronomy.stackexchange.com/a/49116/45954. See also my answer here: https://chemistry.stackexchange.com/a/175028/79678
Ed V
CC BY-SA 4.0
2024-09-17T23:10:50.533Z
Thanks Ed! Yeah I get that each grating order has a range of wavelengths I'm just wondering conceptually why (but maybe you were just responding to Yorik-- in which case disregard this message!)
nittany_astro
CC BY-SA 4.0
2024-09-17T23:59:05.580Z
Well, the fundamental thing a diffraction grating does is diffractively disperse incident light into grating orders, as per the grating equation. I show that equation in one of the figures in the first linked answer. If you look at that equation, you see that it relates the wavelength of light (diffractively exiting the grating) to the exit angle of the diffracted light, other variables, such as grating order, grating ruling density and angle of light incidence being held constant.
Ed V
<spectroscopy><spectra>
Why do higher orders in diffracted light show up as shades of one color?
nittany_astro
# Why do higher orders in diffracted light show up as shades of one color?

(Asked by: username_0 on 2024-09-17)

I'm learning about diffraction gratings and came across this graphic of NEID's spectra: [![spectral format of NEID](https://i.sstatic.net/LhpieeGd.png)](https://i.sstatic.net/LhpieeGd.png). It shows that for each order in the spectra there is a small set of wavelengths (e.g. order 70 has many shades of red but no greens or blues). I was wondering conceptually why each order is associated with one color as opposed to showing up as a rainbow/full spectrum. I know about the grating equation but I'm trying to think of this conceptually as opposed to using math.

**Tags:** spectroscopy, spectra
> **Comments:**
> **username_1**: CCD Stitch boundaries are a technical limitation of CCD sensors and require processing to deal with. Aside from that I know very little, but the source of this image might be helpful. It looks like it has two axes but there is no indication of a key etc. It just seems like an artist's conception of long and short wavelengths  
> **username_0**: This is the source of the image: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9908/1/Design-of-NEID-an-extreme-precision-Doppler-spectrograph-for-WIYN/10.1117/12.2234411.full#_=_ Though I do not think the axes are relevant as I can find another image that demonstrates the same question without axes (and without CCD stitch boundaries for that matter) (see image here https://astronomy.stackexchange.com/questions/38706/a-simple-echelle-spectrograph-for-viewing-solar-spectrum)  
> **username_2**: There is a range of wavelengths in each grating order of the echelle spectrum. See examples of real echelle spectra in my answer here: https://astronomy.stackexchange.com/a/49116/45954. See also my answer here: https://chemistry.stackexchange.com/a/175028/79678  
> **username_0**: Thanks Ed! Yeah I get that each grating order has a range of wavelengths I'm just wondering conceptually why (but maybe you were just responding to Yorik-- in which case disregard this message!)  
> **username_2**: Well, the fundamental thing a diffraction grating does is diffractively disperse incident light into grating orders, as per the grating equation. I show that equation in one of the figures in the first linked answer. If you look at that equation, you see that it relates the wavelength of light (diffractively exiting the grating) to the exit angle of the diffracted light, other variables, such as grating order, grating ruling density and angle of light incidence being held constant.  

---

## 1 answers

### Answer by username_3 on 2024-09-18. Score: 0

The echelle grating spectrometer has two dispersing elements. The first one is a high-resolution echelle grating that disperses light onto a second disperser, a low-resolution cross disperser that disperses in the perpendicular direction. This cross disperser can be either a prism or an echelle grating. The different high orders from the high-res disperser overlap on the cross disperser, with each order naturally slightly offset from the previous one. This design ensures that only certain wavelengths of each order land on the second disperser. In the diagram, only the red light of order 70 falls on the second disperser, and only the blue light of order 160 falls on the disperser, etc. The CCD captures bands of light, each representing a wavelength range with some overlap with adjacent bands.


---

