<blockquote>
<p>Is &quot;monochromatic source&quot; different than &quot;monochromator&quot; in astronomical instrumentation?</p>
</blockquote>
<p>Sure!</p>
<p>&quot;Monochromatic source&quot; is a very general term for any source of radiation of any type that emits a very narrow range of energies. Generally electromagnetic and traditionally optical (vis, near-IR or near-UV), though it can apply from gamma rays all the way to radio.</p>
<p>It could refer to an instrument on Earth (as in your application) or a source of monochromatic radiation in space that one is observing (not your application).</p>
<p>Bottom line: In your context, <strong>a &quot;monochromatic source&quot; is a source of radiation.</strong></p>
<p>A &quot;monochromator&quot; on the other hand is a <em>processor of light</em>. It is a subtractive element, which means it passes a narrow band of energies and rejects (reflects or absorbs) the rest.</p>
<p>Generally what comes to mind is a scanning spectrometer - a device with input and output slits, and a dispersive element (usually a diffraction grating or possibly a prism) and one or more collimating lenses to re-image the input slit at the output slit.</p>
<p>But it could be a Fabry-Perot etalon or even a passive narrow-band filter.</p>
<p>Bottom line: In your context, <strong>a &quot;monochromator&quot; is a processor of radiation from another source.</strong> And that source could be astronomical or a laboratory instrument, but it would be discussed separately</p>

CC BY-SA 4.0
2023-02-26T00:00:34.550Z
2023-02-26T21:09:54.440Z
2023-02-26T21:09:54.440Z
uhoh
<p>I got a comment to replace &quot;monochromator&quot; by &quot;monochromatic source&quot; for a paper on an astronomical instrument. The latter seems less specific for no reason.</p>
<p>Is there an agreement to use one term rather than the other in the context of astronomy?</p>
<p>(I have found no evidence for this but I may have missed something significant)</p>

CC BY-SA 4.0
2023-02-24T14:36:24.603Z
I'll note that "monochromatic source" seems pretty clear to me, while I've never encountered "monochromator" before... (speaking as an astronomer, though not an instrumentalist)
Peter Erwin
CC BY-SA 4.0
2023-02-24T14:46:59.910Z
I think that this basically explains the reason for the comment. I probably assumed that people were familiar with this because it is something I am familiar with. But this may be a very niche thing for astronomy.
PaoloH
CC BY-SA 4.0
2023-02-24T14:49:25.113Z
@PeterErwin - my (non-astronomer) background tells me a monochromator is used to turn a polychromatic source (e.g. a light bulb) into a monochromatic source, particularly if you want the wavelength of choice to be varied.
Jon Custer
CC BY-SA 4.0
2023-02-26T00:06:27.243Z
[Do we need both the nomenclature and terminology tags?](https://astronomy.meta.stackexchange.com/q/897/7982)
uhoh
<terminology><instruments><nomenclature>
Is "monochromatic source" different than "monochromator" in astronomical instrumentation?
PaoloH
# Is "monochromatic source" different than "monochromator" in astronomical instrumentation?

(Asked by: username_0 on 2023-02-24)

I got a comment to replace "monochromator" by "monochromatic source" for a paper on an astronomical instrument. The latter seems less specific for no reason.

Is there an agreement to use one term rather than the other in the context of astronomy?

(I have found no evidence for this but I may have missed something significant)

**Tags:** terminology, instruments, nomenclature
> **Comments:**
> **username_1**: I'll note that "monochromatic source" seems pretty clear to me, while I've never encountered "monochromator" before... (speaking as an astronomer, though not an instrumentalist)  
> **username_0**: I think that this basically explains the reason for the comment. I probably assumed that people were familiar with this because it is something I am familiar with. But this may be a very niche thing for astronomy.  
> **username_2**: @PeterErwin - my (non-astronomer) background tells me a monochromator is used to turn a polychromatic source (e.g. a light bulb) into a monochromatic source, particularly if you want the wavelength of choice to be varied.  
> **username_3**: [Do we need both the nomenclature and terminology tags?](https://astronomy.meta.stackexchange.com/q/897/7982)  

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## 1 answers

### Answer by username_3 on 2023-02-26. Score: 5 | Accepted answer

> Is "monochromatic source" different than "monochromator" in astronomical instrumentation?

Sure!

"Monochromatic source" is a very general term for any source of radiation of any type that emits a very narrow range of energies. Generally electromagnetic and traditionally optical (vis, near-IR or near-UV), though it can apply from gamma rays all the way to radio.

It could refer to an instrument on Earth (as in your application) or a source of monochromatic radiation in space that one is observing (not your application).

Bottom line: In your context, **a "monochromatic source" is a source of radiation.**

A "monochromator" on the other hand is a *processor of light*. It is a subtractive element, which means it passes a narrow band of energies and rejects (reflects or absorbs) the rest.

Generally what comes to mind is a scanning spectrometer - a device with input and output slits, and a dispersive element (usually a diffraction grating or possibly a prism) and one or more collimating lenses to re-image the input slit at the output slit.

But it could be a Fabry-Perot etalon or even a passive narrow-band filter.

Bottom line: In your context, **a "monochromator" is a processor of radiation from another source.** And that source could be astronomical or a laboratory instrument, but it would be discussed separately


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