<p>Cool question!</p>
<p>If you allow for flat mirrors, I've seen five myself. Answer(s) to <a href="https://astronomy.stackexchange.com/q/43357/7982">How did I flip some mirrors around in the dark at 3 AM and change the focal length of a 24 inch Boller and Chivens?</a> show four mirrors and there was one more on the floor below to make the beam horizontal. If you count the aluminized reflective grating as a mirror it was six.</p>
<p><a href="https://i.sstatic.net/vT4gL.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vT4gLm.jpg" alt="title" /></a> ALL IMAGES: click for larger size</p>
<p>For more info, see <a href="https://en.wikipedia.org/wiki/Reflecting_telescope#Nasmyth_and_coud%C3%A9_focus" rel="nofollow noreferrer">Nasmyth and coudé focus</a></p>
<hr />
<p>For more examples of three mirrors built into the telescope itself (as opposed to an eyepiece diagonal for viewing, see answer(s) to</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/27457/7982">How does this telescope work, and what is it for?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/21198/7982">What telescope is this, and how does it work?</a></li>
</ul>
<p><a href="https://i.sstatic.net/oIuG5.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oIuG5m.jpg" alt="title" /></a> <a href="https://i.sstatic.net/vrzFI.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vrzFIm.jpg" alt="title" /></a></p>
<p>For more little mirrors that send light to different instruments, see answers to</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/27758/7982">What is a pupil slicer, and how does it work with anamorphic optics in VLT&#39;s ESPRESSO Echelle spectrograph?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/27912/7982">Where exactly is the modification that first corrected the spherical aberration in Hubble&#39;s primary mirror?</a></li>
</ul>
<p><a href="https://i.sstatic.net/SpN8N.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/SpN8Nm.jpg" alt="title" /></a> <a href="https://i.sstatic.net/bnVuq.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bnVuqm.jpg" alt="title" /></a></p>
<h3>But three or more curved mirrors is interesting</h3>
<p>One example is the Hubble Space Telescope using the <a href="https://en.wikipedia.org/wiki/Wide_Field_and_Planetary_Camera_2" rel="nofollow noreferrer">Wide Field and Planetary Camera 2</a> or WFPC2.</p>
<p>Basically it is another whole telescope inside the Hubble, re-imaging the primary focal plane with a second purely reflective optical Cassegrain system to avoid chromatic aberration. (There would be no way to correct a fused silica lens system from 115 to 1050 nm.)</p>
<p>From <a href="http://what-when-how.com/remote-sensing-from-air-and-space/hubble-the-big-telescope-visible-imagery-remote-sensing/" rel="nofollow noreferrer">what-when-how Hubble: The Big Telescope</a></p>
<blockquote>
<p>Figure 3.36 WFPC2 optics. Light enters the optical train from the main telescope at left.</p>
</blockquote>
<p><a href="https://i.sstatic.net/ljesU.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ljesU.jpg" alt="what-when-how Hubble: The Big Telescope" /></a></p>

CC BY-SA 4.0
2021-05-21T20:06:22.830Z
I suppose increasing the focal length by reflecting the image through the entire length of the telescope again isn't too sensible/too difficult to construct
2080
CC BY-SA 4.0
2021-05-22T00:17:11.977Z
@2080 that's an interesting thought! The f/24 beam (f= 57.6 m / d = 2.4 m) is already on the longish side and the primary focal plane is pretty big so that multiple instruments sample different areas of the focal plane. For some instruments perhaps f/24 is okay, for others like the original [WFPC2](https://en.wikipedia.org/wiki/Wide_Field_and_Planetary_Camera_2) it needed to be lengthened. It's really surprising to find out how little of the Hubble's primary focal plane is used at any one time for astronomy. I think you can ask that as a new question and get some interesting answers!
uhoh
CC BY-SA 4.0
2021-05-20T22:12:32.227Z
2021-05-20T22:33:22.777Z
2021-05-20T22:33:22.777Z
uhoh
<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?</p>
<p><a href="https://i.sstatic.net/pWYlT.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/pWYlT.png" alt="enter image description here" /></a></p>

<telescope>
Do triple or more times reflecting telescopes exist?
2080
# Do triple or more times reflecting telescopes exist?

(Asked by: username_0 on 2021-05-20)

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?

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

**Tags:** telescope

---

## 1 answers

### Answer by username_1 on 2021-05-20. Score: 4 | Accepted answer

Cool question!

If you allow for flat mirrors, I've seen five myself. Answer(s) to [How did I flip some mirrors around in the dark at 3 AM and change the focal length of a 24 inch Boller and Chivens?](https://astronomy.stackexchange.com/q/43357/7982) show four mirrors and there was one more on the floor below to make the beam horizontal. If you count the aluminized reflective grating as a mirror it was six.

[![title](https://i.sstatic.net/vT4gLm.jpg)](https://i.sstatic.net/vT4gL.jpg) ALL IMAGES: click for larger size

For more info, see [Nasmyth and coudé focus](https://en.wikipedia.org/wiki/Reflecting_telescope#Nasmyth_and_coud%C3%A9_focus)

---

For more examples of three mirrors built into the telescope itself (as opposed to an eyepiece diagonal for viewing, see answer(s) to

* [How does this telescope work, and what is it for?](https://astronomy.stackexchange.com/q/27457/7982)
* [What telescope is this, and how does it work?](https://astronomy.stackexchange.com/q/21198/7982)

[![title](https://i.sstatic.net/oIuG5m.jpg)](https://i.sstatic.net/oIuG5.jpg) [![title](https://i.sstatic.net/vrzFIm.jpg)](https://i.sstatic.net/vrzFI.jpg)

For more little mirrors that send light to different instruments, see answers to

* [What is a pupil slicer, and how does it work with anamorphic optics in VLT's ESPRESSO Echelle spectrograph?](https://astronomy.stackexchange.com/q/27758/7982)
* [Where exactly is the modification that first corrected the spherical aberration in Hubble's primary mirror?](https://astronomy.stackexchange.com/q/27912/7982)

[![title](https://i.sstatic.net/SpN8Nm.jpg)](https://i.sstatic.net/SpN8N.jpg) [![title](https://i.sstatic.net/bnVuqm.jpg)](https://i.sstatic.net/bnVuq.jpg)

### But three or more curved mirrors is interesting

One example is the Hubble Space Telescope using the [Wide Field and Planetary Camera 2](https://en.wikipedia.org/wiki/Wide_Field_and_Planetary_Camera_2) or WFPC2.

Basically it is another whole telescope inside the Hubble, re-imaging the primary focal plane with a second purely reflective optical Cassegrain system to avoid chromatic aberration. (There would be no way to correct a fused silica lens system from 115 to 1050 nm.)

From [what-when-how Hubble: The Big Telescope](http://what-when-how.com/remote-sensing-from-air-and-space/hubble-the-big-telescope-visible-imagery-remote-sensing/)

> Figure 3.36 WFPC2 optics. Light enters the optical train from the main telescope at left.

[![what-when-how Hubble: The Big Telescope](https://i.sstatic.net/ljesU.jpg)](https://i.sstatic.net/ljesU.jpg)

> **Comments:**
> **username_0**: I suppose increasing the focal length by reflecting the image through the entire length of the telescope again isn't too sensible/too difficult to construct  
> **username_1**: @2080 that's an interesting thought! The f/24 beam (f= 57.6 m / d = 2.4 m) is already on the longish side and the primary focal plane is pretty big so that multiple instruments sample different areas of the focal plane. For some instruments perhaps f/24 is okay, for others like the original [WFPC2](https://en.wikipedia.org/wiki/Wide_Field_and_Planetary_Camera_2) it needed to be lengthened. It's really surprising to find out how little of the Hubble's primary focal plane is used at any one time for astronomy. I think you can ask that as a new question and get some interesting answers!  

---

