<p>If your willing to accept more than 2 discontinuous mirrors, the <a href="https://en.wikipedia.org/wiki/Three-mirror_anastigmat" rel="noreferrer">Three Mirror Anastigmat</a> has 4 passes along some/most of the overall tube length. An early working prototype example (which I've actually seen in person many years ago) was built at the University of Cambridge by Dr. Roderick Willstrop. The Institute of Astronomy has a <a href="https://www.ast.cam.ac.uk/about/three-mirror.telescope" rel="noreferrer">page on the Three Mirror Telescope</a> which includes the following optical diagram:</p>
<p><a href="https://i.sstatic.net/sf8f8.png" rel="noreferrer"><img src="https://i.sstatic.net/sf8f8.png" alt="Optical layout of the Three Mirror Anastigmat (Willstrop) telescope at Cambridge" /></a></p>
<p>The 0.5 meter 3 Mirror Telescope (3MT) has a focal length of 0.8 meters, in an overall tube length of 1.2 meters producing a very compact telescope with a large field of view (5 degrees in diameter) with good image quality (&lt;0.33&quot; across the whole field of view). The quality of the site at Cambridge was not ideal for building on the progress of the prototype and a proposal for building a larger version at a better site was declined by the UK science funding agency of the time (SERC, which became PPARC and then STFC) in favor of buying into the Gemini collaboration of two 8 meter telescopes.</p>
<p>Some constructed or in construction examples of three mirror anastigmat include:</p>
<ul>
<li>James Webb Space Telescope (see Figure 2 of <a href="https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-telescope" rel="noreferrer">this page on the JWST Telescope</a>)</li>
<li>The Simonyi Survey Telescope of the Vera C. Rubin Observatory which will carry out the Legacy Survey of Space and Time (LSST) has M1 and M3 ground into the same piece of glass (see Figure 7 of the <a href="https://arxiv.org/pdf/0805.2366.pdf" rel="noreferrer">LSST overview paper</a>) with a separate secondary M2 mirror and prime focus camera</li>
<li>The planned ESO Extremely Large Telescope will use 5 mirrors in a folded three mirror anastigmat design along with 2 flat mirrors which will have 5 bounces along all or part of the overall &quot;tube&quot; structure before exiting to a Nasmyth focus; see the not very clear diagram below:   <a href="https://i.sstatic.net/5KYlU.jpg" rel="noreferrer"><img src="https://i.sstatic.net/5KYlU.jpg" alt="optical diagram of ESOs Extremely Large Telescope" /></a> from the ESO ELT <a href="https://www.eso.org/sci/facilities/eelt/telescope/design/" rel="noreferrer">optical diagram</a> page, somewhat better diagram in  Figure 4 of <a href="https://arxiv.org/pdf/1808.02693.pdf" rel="noreferrer">Hippler 2018</a>.</li>
</ul>

CC BY-SA 4.0
2021-05-24T17:28:12.150Z
fancy! thank you :)
2080
CC BY-SA 4.0
2021-05-24T23:04:56.573Z
related: [What is a quaternary mirror and why does the E-ELT need one?](https://astronomy.stackexchange.com/q/15005/7982)
uhoh
CC BY-SA 4.0
2021-05-24T17:10:27.973Z
2021-05-24T17:10:27.973Z
astrosnapper
<p>Refractors only use the length of the telescope once, reflectors twice, catadioptric telescopes like those of the Schmidt-Cassegrain design three times. Have telescopes been built that reflect the incoming light at least once more from one end of the telescope to another?</p>
<p><a href="https://i.sstatic.net/nrugT.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/nrugT.png" alt="enter image description here" /></a></p>
<p>Say, by adding a tertiary mirror which reflects the light forward again, towards a small quarternary mirror in front of the secondary, which ultimately directs the light though a small hole in the tertiary (and primary) mirror towards the eyepiece?</p>
<p><strong>Note:</strong> The above description makes this question different from the similar-sounding <a href="https://astronomy.stackexchange.com/q/43900/7982">Do triple or more times reflecting telescopes exist?</a></p>

CC BY-SA 4.0
2021-05-23T21:33:36.067Z
Do [Nasmyth and coude](http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_coude.html) foci count ?
astrosnapper
CC BY-SA 4.0
2021-05-23T22:05:54.830Z
@astrosnapper Not quite, these are more fitting for my previous question: https://astronomy.stackexchange.com/questions/43900/do-triple-or-more-times-reflecting-telescopes-exist In this one I specifically want to know if the described setup exists
2080
CC BY-SA 4.0
2021-05-24T14:32:16.557Z
I remember (but cannot track down) an ATM folded-refractor in Sky and Telescope that had three mirrors for a kind of quadrilateral light path, designed to minimize the off-axis aberration of more conventional folded refractors.
antlersoft
<telescope><optics>
Do telescopes exist that reflect the incoming light more than three times along their length?
2080
# Do telescopes exist that reflect the incoming light more than three times along their length?

(Asked by: username_0 on 2021-05-23)

Refractors only use the length of the telescope once, reflectors twice, catadioptric telescopes like those of the Schmidt-Cassegrain design three times. Have telescopes been built that reflect the incoming light at least once more from one end of the telescope to another?

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

Say, by adding a tertiary mirror which reflects the light forward again, towards a small quarternary mirror in front of the secondary, which ultimately directs the light though a small hole in the tertiary (and primary) mirror towards the eyepiece?

**Note:** The above description makes this question different from the similar-sounding [Do triple or more times reflecting telescopes exist?](https://astronomy.stackexchange.com/q/43900/7982)

**Tags:** telescope, optics
> **Comments:**
> **username_1**: Do [Nasmyth and coude](http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/telescopes/phy217_tel_coude.html) foci count ?  
> **username_0**: @astrosnapper Not quite, these are more fitting for my previous question: https://astronomy.stackexchange.com/questions/43900/do-triple-or-more-times-reflecting-telescopes-exist In this one I specifically want to know if the described setup exists  
> **username_2**: I remember (but cannot track down) an ATM folded-refractor in Sky and Telescope that had three mirrors for a kind of quadrilateral light path, designed to minimize the off-axis aberration of more conventional folded refractors.  

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## 1 answers

### Answer by username_1 on 2021-05-24. Score: 8 | Accepted answer

If your willing to accept more than 2 discontinuous mirrors, the [Three Mirror Anastigmat](https://en.wikipedia.org/wiki/Three-mirror_anastigmat) has 4 passes along some/most of the overall tube length. An early working prototype example (which I've actually seen in person many years ago) was built at the University of Cambridge by Dr. Roderick Willstrop. The Institute of Astronomy has a [page on the Three Mirror Telescope](https://www.ast.cam.ac.uk/about/three-mirror.telescope) which includes the following optical diagram:

[![Optical layout of the Three Mirror Anastigmat (Willstrop) telescope at Cambridge](https://i.sstatic.net/sf8f8.png)](https://i.sstatic.net/sf8f8.png)

The 0.5 meter 3 Mirror Telescope (3MT) has a focal length of 0.8 meters, in an overall tube length of 1.2 meters producing a very compact telescope with a large field of view (5 degrees in diameter) with good image quality (<0.33" across the whole field of view). The quality of the site at Cambridge was not ideal for building on the progress of the prototype and a proposal for building a larger version at a better site was declined by the UK science funding agency of the time (SERC, which became PPARC and then STFC) in favor of buying into the Gemini collaboration of two 8 meter telescopes.

Some constructed or in construction examples of three mirror anastigmat include:

* James Webb Space Telescope (see Figure 2 of [this page on the JWST Telescope](https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-telescope))
* The Simonyi Survey Telescope of the Vera C. Rubin Observatory which will carry out the Legacy Survey of Space and Time (LSST) has M1 and M3 ground into the same piece of glass (see Figure 7 of the [LSST overview paper](https://arxiv.org/pdf/0805.2366.pdf)) with a separate secondary M2 mirror and prime focus camera
* The planned ESO Extremely Large Telescope will use 5 mirrors in a folded three mirror anastigmat design along with 2 flat mirrors which will have 5 bounces along all or part of the overall "tube" structure before exiting to a Nasmyth focus; see the not very clear diagram below: [![optical diagram of ESOs Extremely Large Telescope](https://i.sstatic.net/5KYlU.jpg)](https://i.sstatic.net/5KYlU.jpg) from the ESO ELT [optical diagram](https://www.eso.org/sci/facilities/eelt/telescope/design/) page, somewhat better diagram in Figure 4 of [Hippler 2018](https://arxiv.org/pdf/1808.02693.pdf).

> **Comments:**
> **username_0**: fancy! thank you :)  
> **username_3**: related: [What is a quaternary mirror and why does the E-ELT need one?](https://astronomy.stackexchange.com/q/15005/7982)  

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