<p>When reading <a href="https://astronomy.stackexchange.com/questions/40984/why-was-stdr56-discovered-only-now">Why was StDr56 discovered only now?</a> and its answers and comments, I realized that refractor telescopes, and in particular the <a href="https://en.wikipedia.org/wiki/Dragonfly_Telephoto_Array" rel="nofollow noreferrer">Dragonfly Telephoto Array</a> are a great tool to find nebula and other faint objects. The proud parents at the <a href="http://www.dunlap.utoronto.ca/instrumentation/dragonfly/" rel="nofollow noreferrer">Dunlap Institute</a> summarize their technique nicely:</p>
<blockquote>
<p>Dragonfly images a galaxy through multiple lenses simultaneously—akin to a dragonfly’s compound eye—enabling further removal of unwanted light. The result is an image in which extremely faint galaxy structure is visible.</p>
</blockquote>
<p>Why are Earth-bound mirror telescopes not able to see such objects? To use <a href="https://astronomy.stackexchange.com/users/7982/uhoh">uhoh</a>'s words:</p>
<blockquote>
<p>[T]he back story there is that refractors were better suited than reflectors because nano-roughness of silvered mirrors produces a faint haze of scattered starlight that competes with natural skyglow to &quot;fog&quot; exposures and hide objects with low surface brightness.</p>
</blockquote>
<p>This finally brings me to my question: <strong>How can this haze originating from the mirror telescope itself</strong> (and <em>not</em> from atmospheric effects) <strong>be quantified?</strong> Why does the nano-roughness of a mirror disturbes the image more than <a href="https://en.wikipedia.org/wiki/Optical_aberration" rel="nofollow noreferrer">optical abberation</a> of the lenses in a refractor telescope? Is it really only the nano-roughness? Or is it maybe also the support of the secondary mirror <a href="https://www.physicsforums.com/threads/why-do-pictures-of-stars-have-4-cardinal-points.969480/" rel="nofollow noreferrer">which is responsible for the &quot;spikes&quot; in pictures of stars</a>?</p>
<h3>References</h3>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/39572/7982">Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light)</a></li>
<li><a href="https://astronomy.stackexchange.com/questions/34235/what-are-the-benefits-of-a-large-refracting-telescope">What are the benefits of a large refracting telescope?</a></li>
<li><a href="https://astronomy.stackexchange.com/questions/39572/deciding-optical-factors-between-a-refractive-and-reflective-space-telescope-opt">Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light)</a></li>
<li>Shany Danieli <em>et al.</em> <a href="https://arxiv.org/abs/1910.14045" rel="nofollow noreferrer">The Dragonfly Wide Field Survey. I. Telescope, Survey Design and Data Characterization</a></li>
<li>Deborah M. Lokhorst <em>et al.</em> <a href="https://arxiv.org/abs/2010.00686" rel="nofollow noreferrer">Wide-field ultra-narrow-bandpass imaging with the Dragonfly Telephoto Array</a></li>
<li>Roberto Abraham <em>et al.</em> <a href="https://arxiv.org/abs/1612.06415" rel="nofollow noreferrer">Future Prospects: Deep Imaging of Galaxy Outskirts using Telescopes Large and Small</a></li>
</ul>

CC BY-SA 4.0
2021-01-22T23:20:33.130Z
@uhoh This is the question I promised to write :-)
B--rian
CC BY-SA 4.0
2021-01-22T23:29:19.120Z
Can anyone provide a reference for the idea that mirrors are necessarily rougher than lenses?
Mike G
CC BY-SA 4.0
2021-01-23T01:35:16.203Z
@MikeG The [last reference](https://arxiv.org/abs/1612.06415) cited in the question, page 9 for example, and references therein.
uhoh
CC BY-SA 4.0
2021-01-23T02:40:33.620Z
I guess [Abraham et al. 2017](https://ui.adsabs.harvard.edu/abs/2017ASSL..434..333A/abstract) meant to cite [Sandin 2014](https://ui.adsabs.harvard.edu/abs/2014A%26A...567A..97S/abstract)...which cites [Abraham and van Dokkum 2014](https://ui.adsabs.harvard.edu/abs/2014PASP..126...55A/abstract)...hmm. Still not seeing justification for a sweeping "refractors > reflectors."
Mike G
<telescope><optics><nebula><refractor-telescope>
Can the "haze" of a mirror telescope be quantified?
B--rian
# Can the "haze" of a mirror telescope be quantified?

(Asked by: username_0 on 2021-01-22)

When reading [Why was StDr56 discovered only now?](https://astronomy.stackexchange.com/questions/40984/why-was-stdr56-discovered-only-now) and its answers and comments, I realized that refractor telescopes, and in particular the [Dragonfly Telephoto Array](https://en.wikipedia.org/wiki/Dragonfly_Telephoto_Array) are a great tool to find nebula and other faint objects. The proud parents at the [Dunlap Institute](http://www.dunlap.utoronto.ca/instrumentation/dragonfly/) summarize their technique nicely:

> Dragonfly images a galaxy through multiple lenses simultaneously—akin to a dragonfly’s compound eye—enabling further removal of unwanted light. The result is an image in which extremely faint galaxy structure is visible.

Why are Earth-bound mirror telescopes not able to see such objects? To use [uhoh](https://astronomy.stackexchange.com/users/7982/uhoh)'s words:

> [T]he back story there is that refractors were better suited than reflectors because nano-roughness of silvered mirrors produces a faint haze of scattered starlight that competes with natural skyglow to "fog" exposures and hide objects with low surface brightness.

This finally brings me to my question: **How can this haze originating from the mirror telescope itself** (and *not* from atmospheric effects) **be quantified?** Why does the nano-roughness of a mirror disturbes the image more than [optical abberation](https://en.wikipedia.org/wiki/Optical_aberration) of the lenses in a refractor telescope? Is it really only the nano-roughness? Or is it maybe also the support of the secondary mirror [which is responsible for the "spikes" in pictures of stars](https://www.physicsforums.com/threads/why-do-pictures-of-stars-have-4-cardinal-points.969480/)?

### References

* [Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light)](https://astronomy.stackexchange.com/q/39572/7982)
* [What are the benefits of a large refracting telescope?](https://astronomy.stackexchange.com/questions/34235/what-are-the-benefits-of-a-large-refracting-telescope)
* [Deciding optical factors between a refractive and reflective space telescope optics as a function of aperture? (visible light)](https://astronomy.stackexchange.com/questions/39572/deciding-optical-factors-between-a-refractive-and-reflective-space-telescope-opt)
* Shany Danieli *et al.* [The Dragonfly Wide Field Survey. I. Telescope, Survey Design and Data Characterization](https://arxiv.org/abs/1910.14045)
* Deborah M. Lokhorst *et al.* [Wide-field ultra-narrow-bandpass imaging with the Dragonfly Telephoto Array](https://arxiv.org/abs/2010.00686)
* Roberto Abraham *et al.* [Future Prospects: Deep Imaging of Galaxy Outskirts using Telescopes Large and Small](https://arxiv.org/abs/1612.06415)

**Tags:** telescope, optics, nebula, refractor-telescope
> **Comments:**
> **username_0**: @uhoh This is the question I promised to write :-)  
> **username_1**: Can anyone provide a reference for the idea that mirrors are necessarily rougher than lenses?  
> **username_2**: @MikeG The [last reference](https://arxiv.org/abs/1612.06415) cited in the question, page 9 for example, and references therein.  
> **username_1**: I guess [Abraham et al. 2017](https://ui.adsabs.harvard.edu/abs/2017ASSL..434..333A/abstract) meant to cite [Sandin 2014](https://ui.adsabs.harvard.edu/abs/2014A%26A...567A..97S/abstract)...which cites [Abraham and van Dokkum 2014](https://ui.adsabs.harvard.edu/abs/2014PASP..126...55A/abstract)...hmm. Still not seeing justification for a sweeping "refractors > reflectors."  

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## 0 answers

