<p>The spectral reflectance curve depends on the coatings applied.  For 'scopes used by people there is no point in maintaining reflectivity below 450 or above 700 nm.  You idea of &quot;visible&quot; range is, I fear somewhat optimistic.  You may be better off with an uncoated Al mirror (spectral curves easily found on the net) and giving up some signal in the photopic range.  -- or with a nice Ag overcoat, perhaps.</p>
<h2>edit</h2>
<p>Per request, some data on uncoated Al.
From   <a href="https://laserbeamproducts.wordpress.com/2014/06/19/reflectivity-of-aluminium-uv-visible-and-infrared/" rel="nofollow noreferrer">laserbeamproducts</a>,</p>
<blockquote>
<p>REFLECTIVITY OF PURE UNCOATED ALUMINIUM AT NORMAL INCIDENCE</p>
</blockquote>
<pre><code>Wavelength            % Reflectivity 
248nm                      92.6 
400nm                      92.0 
532nm                      91.6 
633nm                      90.7 
800nm                      86.8 
900nm                      89.0 
1um                          94.0 
3um                          98.0
10.6um                     98.7 
20um                        99.0 
100um                      99.4
</code></pre>
<p>An image published (by the author) at <a href="https://www.researchgate.net/figure/Spectral-reflectance-of-aluminum-and-various-other-metals-O-Elsevier-Reprinted-with_fig1_283437763" rel="nofollow noreferrer">Researchgate</a></p>
<p><a href="https://i.sstatic.net/Nu3Rc.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Nu3Rc.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2018-10-18T18:01:09.377Z
This is on a telescope feeding either an optical CCD imager or an optical low resolution spectrograph which covers 320-1000nm so my range is only slightly beyond this. I recognize that no mirror coating is perfect and won't have great sensitivity everywhere. Can you mod the answer and provide an example uncoated Al data link for the education of all...
astrosnapper
CC BY-SA 4.0
2018-10-19T10:12:04.640Z
In *teloscopy-speak*, the term "visible light" is also used to refer to any chunk of wavelengths not extending deep enough into UV to have to call it UV-vis, nor deep enough into IR to call it vis-IR. random example: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6636 "visible coating" as opposed to their IR or UV coating
uhoh
CC BY-SA 4.0
2018-10-19T12:08:24.047Z
@uhoh not universally, sadly.  I know of a variety of sensors labelled "VNIR" (vis- near IR) which cut out at maybe 950 nm, for example.
Carl Witthoft
CC BY-SA 4.0
2018-10-19T12:09:12.873Z
@astrosnapper  am I correct that the CCD has a much smaller sensitivity range, and only your specialized spectrograph device covers the full range?
Carl Witthoft
CC BY-SA 4.0
2018-10-19T12:18:18.813Z
@astrosnapper  added graph and tabular data
Carl Witthoft
CC BY-SA 4.0
2018-10-19T20:53:28.247Z
@CarlWitthoft No both CCDs have about the same coverage, the spectrograph is better in the blue, the imager is better in the red. I've plotted the QE curves (also hand measured off plots) at https://cloud.lco.global/s/YywFjcCXCFr9WDw Other factors come in of course such as the atmosphere (also plotted), the grating/filter used and the color of the source in determining how much sensitivity there is.
astrosnapper
CC BY-SA 4.0
2018-10-18T17:34:20.647Z
2018-10-19T12:18:00.247Z
2020-06-17T09:47:40.653Z
Carl Witthoft
<p>I came across an article in the <a href="https://www.noao.edu/noao/noaonews/oct18/118news.pdf" rel="nofollow noreferrer">NOAO 2018 October Newsletter</a> which discussed recoating of the 4-meter Blanco primary in Chile. This contained before, after and theoretical reflectivity data. I contacted one of the authors of the article for the source of the theoretical data who sent me a scan of the pages from R. N. Wilson's "Reflecting Telescope Optics II", page 426. I transcribed the data from the table for Alumin(i)um (Al) and Silver (Ag) and below is a plot of the Al and Ag data in the visible range:</p>

<p><a href="https://i.sstatic.net/jYiIc.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jYiIc.png" alt="Reflectivity of Aluminum (Al) and Silver (Ag) mirrors"></a></p>

<p>(I've made more plots and the data files available in a <a href="https://adventuresinprecision.space/2019/01/28/mirror-reflectivity-data/" rel="nofollow noreferrer">post</a> on my blog)</p>

CC BY-SA 4.0
2019-01-31T05:57:37.990Z
That's quite cool! Any chance you might add an answer to [Do primary mirrors in large observatories undergo regular removal and re-coating of the aluminum? Why?](https://astronomy.stackexchange.com/q/16607/7982) Also the outstanding issue in [When did “resilvering” large telescope mirrors actually refer to aluminization, **and why was it necessary?**](https://astronomy.stackexchange.com/q/16583/7982) is the reason for why regular coating of aluminum mirrors would be necessary since aluminum forms a hard, impermeable native oxide quickly. Maybe the *scattering data* in your link addresses that?
uhoh
CC BY-SA 4.0
2019-01-28T19:05:17.723Z
2019-01-28T19:05:17.723Z
astrosnapper
<p>I am trying to find a source of mirror reflectivity versus wavelength for the typical protected overcoat Aluminum mirrors used in astronomy. I need this to cover the whole visible range (300-1200nm) and ideally machine-readable. I've only been able to find part coverage of the wavelength range or single spot measurements at a specific wavelength. </p>

<p>The best I've managed is <a href="https://www.edmundoptics.com/resources/application-notes/optics/metallic-mirror-coatings/" rel="nofollow noreferrer">this page at Edmund Optics</a> which has (small) plots but no table or downloadable file.</p>

<telescope><light><mirror>
Source of mirror reflectivity data
astrosnapper
# Source of mirror reflectivity data

(Asked by: username_0 on 2018-10-18)

I am trying to find a source of mirror reflectivity versus wavelength for the typical protected overcoat Aluminum mirrors used in astronomy. I need this to cover the whole visible range (300-1200nm) and ideally machine-readable. I've only been able to find part coverage of the wavelength range or single spot measurements at a specific wavelength.

The best I've managed is [this page at Edmund Optics](https://www.edmundoptics.com/resources/application-notes/optics/metallic-mirror-coatings/) which has (small) plots but no table or downloadable file.

**Tags:** telescope, light, mirror


---

## 2 answers

### Answer by username_1 on 2018-10-18. Score: 3

The spectral reflectance curve depends on the coatings applied.  For 'scopes used by people there is no point in maintaining reflectivity below 450 or above 700 nm.  You idea of "visible" range is, I fear somewhat optimistic.  You may be better off with an uncoated Al mirror (spectral curves easily found on the net) and giving up some signal in the photopic range.  -- or with a nice Ag overcoat, perhaps.
edit
Per request, some data on uncoated Al.
From   laserbeamproducts,

REFLECTIVITY OF PURE UNCOATED ALUMINIUM AT NORMAL INCIDENCE

Wavelength            % Reflectivity 
248nm                      92.6 
400nm                      92.0 
532nm                      91.6 
633nm                      90.7 
800nm                      86.8 
900nm                      89.0 
1um                          94.0 
3um                          98.0
10.6um                     98.7 
20um                        99.0 
100um                      99.4

An image published (by the author) at Researchgate

> **Comments:**
> **username_0**: This is on a telescope feeding either an optical CCD imager or an optical low resolution spectrograph which covers 320-1000nm so my range is only slightly beyond this. I recognize that no mirror coating is perfect and won't have great sensitivity everywhere. Can you mod the answer and provide an example uncoated Al data link for the education of all...  
> **username_2**: In *teloscopy-speak*, the term "visible light" is also used to refer to any chunk of wavelengths not extending deep enough into UV to have to call it UV-vis, nor deep enough into IR to call it vis-IR. random example: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6636 "visible coating" as opposed to their IR or UV coating  
> **username_1**: @username_2 not universally, sadly.  I know of a variety of sensors labelled "VNIR" (vis- near IR) which cut out at maybe 950 nm, for example.  
> **username_1**: @username_0  am I correct that the CCD has a much smaller sensitivity range, and only your specialized spectrograph device covers the full range?  
> **username_1**: @username_0  added graph and tabular data  
> **username_0**: @CarlWitthoft No both CCDs have about the same coverage, the spectrograph is better in the blue, the imager is better in the red. I've plotted the QE curves (also hand measured off plots) at https://cloud.lco.global/s/YywFjcCXCFr9WDw Other factors come in of course such as the atmosphere (also plotted), the grating/filter used and the color of the source in determining how much sensitivity there is.  

---

### Answer by username_0 on 2019-01-28. Score: 2

I came across an article in the [NOAO 2018 October Newsletter](https://www.noao.edu/noao/noaonews/oct18/118news.pdf) which discussed recoating of the 4-meter Blanco primary in Chile. This contained before, after and theoretical reflectivity data. I contacted one of the authors of the article for the source of the theoretical data who sent me a scan of the pages from R. N. Wilson's "Reflecting Telescope Optics II", page 426. I transcribed the data from the table for Alumin(i)um (Al) and Silver (Ag) and below is a plot of the Al and Ag data in the visible range:

[![Reflectivity of Aluminum (Al) and Silver (Ag) mirrors](https://i.sstatic.net/jYiIc.png)](https://i.sstatic.net/jYiIc.png)

(I've made more plots and the data files available in a [post](https://adventuresinprecision.space/2019/01/28/mirror-reflectivity-data/) on my blog)

> **Comments:**
> **username_2**: That's quite cool! Any chance you might add an answer to [Do primary mirrors in large observatories undergo regular removal and re-coating of the aluminum? Why?](https://astronomy.stackexchange.com/q/16607/7982) Also the outstanding issue in [When did “resilvering” large telescope mirrors actually refer to aluminization, **and why was it necessary?**](https://astronomy.stackexchange.com/q/16583/7982) is the reason for why regular coating of aluminum mirrors would be necessary since aluminum forms a hard, impermeable native oxide quickly. Maybe the *scattering data* in your link addresses that?  

---

