<h3>Yes! Those with a <a href="https://astronomy.stackexchange.com/a/43363/7982">Coudé focus</a> and others</h3>
<p>The following</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/a/49247/7982">this answer to <em>Why aren't reflector telescopes built with an offset secondary mirror?</em></a></li>
<li><a href="https://astronomy.stackexchange.com/a/43977/7982">this answer to <em>Do telescopes exist that reflect the incoming light more than three times along their length?</em></a></li>
<li><a href="https://astronomy.stackexchange.com/a/43363/7982">this answer to <em>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?</em></a></li>
<li><a href="https://astronomy.stackexchange.com/a/47067/7982">this answer to <em>Why is the Nancy Grace Roman Space Telescope's field of view shaped like a PacMan &quot;ghost&quot;?</em></a></li>
</ul>
<p>show and link to a wide variety of reflecting telescope optical designs that include negative focal length mirrors and off-axis deflection. (Check them out, but two examples are shown below).</p>
<p>As for a proper, classical Newtonian with its truly 45° diagonal, <strong>it will certainly have been tried by some amateur, somewhere, sometime, if for no other reason than they didn't have access to a primary mirror (blank, or figured) with a hole in the center.</strong></p>
<p><strong>Another reason would be viewing convenience</strong> - that side-viewing option is pretty handy when your DIY portable telescope is big and heavy and for mechanical reasons a tall mount high enough to make bottom viewing with an external diagonal (inverting or fancy non-inverting) a pain.</p>
<p>Just for example, LOptics.com's <a href="http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html" rel="nofollow noreferrer">Building an open-tubed 12.5&quot; F/12.5 Classical Cassegrain By Michael E. Lockwood</a></p>
<p><a href="https://i.sstatic.net/bGVIh.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bGVIh.jpg" alt="LOptics.com's &quot;Building an open-tubed 12.5&quot; F/12.5 Classical Cassegrain&quot; By Michael E. Lockwood http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html" /></a></p>
<p><a href="https://i.sstatic.net/xsgGk.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xsgGk.jpg" alt="LOptics.com's &quot;Building an open-tubed 12.5&quot; F/12.5 Classical Cassegrain&quot; By Michael E. Lockwood http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html" /></a></p>
<hr />
<hr />
<p>from <a href="https://astronomy.stackexchange.com/a/49247/7982">https://astronomy.stackexchange.com/a/49247/7982</a></p>
<blockquote>
<p>From <a href="http://dx.doi.org/10.1117/1.OE.53.3.031311" rel="nofollow noreferrer">Design and development of a freeform active mirror for an astronomy application</a> (also <a href="https://www.researchgate.net/publication/259645665_Design_and_development_of_a_freeform_active_mirror_for_an_astronomy_application" rel="nofollow noreferrer">here</a>):</p>
<p><a href="https://i.sstatic.net/Bs0KT.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Bs0KT.png" alt="&quot;Design and development of a freeform active mirror for an astronomy application&quot; http://dx.doi.org/10.1117/1.OE.53.3.031311" /></a></p>
</blockquote>
<hr />
<p>From <a href="https://astronomy.stackexchange.com/a/43363/7982">https://astronomy.stackexchange.com/a/43363/7982</a></p>
<p><a href="https://i.sstatic.net/gKt9l.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/gKt9l.png" alt="enter image description here" /></a></p>
<hr />
<p>from <a href="https://astronomy.stackexchange.com/a/43977/7982">https://astronomy.stackexchange.com/a/43977/7982</a></p>
<p><a href="https://i.sstatic.net/5KYlU.jpg" rel="nofollow 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="nofollow noreferrer">optical diagram</a> page, somewhat better diagram in  Figure 4 of <a href="https://arxiv.org/pdf/1808.02693.pdf" rel="nofollow noreferrer">Hippler 2018</a>.</p>

CC BY-SA 4.0
2023-08-23T06:20:29.143Z
It looks like. But this is closer to the Cassegrain reflecting telescope with an additional diagonal mirror. The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror.
Imyaf
CC BY-SA 4.0
2023-08-23T09:37:06.227Z
@Imyaf well "like a Newtonian" is not a very well-defined term. You haven't laid out your criteria in the question so answers have to judge for themselves what "like a Newtonian" does and does not mean. It would have been better if your comment "The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror" was in the original question if that's what you were thinking.
uhoh
CC BY-SA 4.0
2023-08-21T23:37:32.037Z
2023-08-21T23:37:32.037Z
uhoh
<p>A Cassegrain reflecting telescope contains a negative optical element between the main mirror and its focus. This increases it's focal length.
<a href="https://i.sstatic.net/Wsf25.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Wsf25.png" alt="enter image description here" /></a></p>
<p>Is there a reflecting telescope like as a Newtonian telescope but with the negative lens before the diagonal mirror?
<a href="https://i.sstatic.net/04h8W.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/04h8W.png" alt="enter image description here" /></a></p>
<p>This question is not about a Barlow lens.</p>

CC BY-SA 4.0
2023-08-20T10:04:42.887Z
Optically it makes no difference whether you place such component in the place you indicate or outside the tube after the secondary mirror. It makes a difference for the usable aperture as it would require a larger secondary mirror or obstruction that the primary light does not pass the negative lens, thus it would reduce the light gathering capability of the telescope. Thus: likely not, as it only has disadvantages and not advantage (as far as I see)
planetmaker
CC BY-SA 4.0
2023-08-21T23:39:34.517Z
@planetmaker "likely not, as it only has disadvantages and not advantage" I don't quite [follow your reasoning](https://astronomy.stackexchange.com/a/54536/7982) can you elaborate?
uhoh
CC BY-SA 4.0
2023-08-22T00:34:29.257Z
I seem to remember a commercial telescope in the 1980s or 1990s that had this optical design. A cheap brand, too, like Tasco or something similar. The mirror was spherical instead of parabolic, and the lens was there to correct the spherical aberration.
Pierre Paquette
<telescope><newtonian-telescope><telescope-lens><mirror><cassegrain-telescope>
Is there a reflecting telescope like as a Newtonian telescope but with a negative lens before the diagonal mirror?
Imyaf
# Is there a reflecting telescope like as a Newtonian telescope but with a negative lens before the diagonal mirror?

(Asked by: username_0 on 2023-08-20)

A Cassegrain reflecting telescope contains a negative optical element between the main mirror and its focus. This increases it's focal length.
[![enter image description here](https://i.sstatic.net/Wsf25.png)](https://i.sstatic.net/Wsf25.png)

Is there a reflecting telescope like as a Newtonian telescope but with the negative lens before the diagonal mirror?
[![enter image description here](https://i.sstatic.net/04h8W.png)](https://i.sstatic.net/04h8W.png)

This question is not about a Barlow lens.

**Tags:** telescope, newtonian-telescope, telescope-lens, mirror, cassegrain-telescope

> **Comments:**
> **username_1**: Optically it makes no difference whether you place such component in the place you indicate or outside the tube after the secondary mirror. It makes a difference for the usable aperture as it would require a larger secondary mirror or obstruction that the primary light does not pass the negative lens, thus it would reduce the light gathering capability of the telescope. Thus: likely not, as it only has disadvantages and not advantage (as far as I see)  
> **username_2**: @username_1 "likely not, as it only has disadvantages and not advantage" I don't quite [follow your reasoning](https://astronomy.stackexchange.com/a/54536/7982) can you elaborate?  
> **username_3**: I seem to remember a commercial telescope in the 1980s or 1990s that had this optical design. A cheap brand, too, like Tasco or something similar. The mirror was spherical instead of parabolic, and the lens was there to correct the spherical aberration.  

---

## 1 answers

### Answer by username_2 on 2023-08-21. Score: 4

### Yes! Those with a [Coudé focus](https://astronomy.stackexchange.com/a/43363/7982) and others

The following

* [this answer to *Why aren't reflector telescopes built with an offset secondary mirror?*](https://astronomy.stackexchange.com/a/49247/7982)
* [this answer to *Do telescopes exist that reflect the incoming light more than three times along their length?*](https://astronomy.stackexchange.com/a/43977/7982)
* [this answer 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/a/43363/7982)
* [this answer to *Why is the Nancy Grace Roman Space Telescope's field of view shaped like a PacMan "ghost"?*](https://astronomy.stackexchange.com/a/47067/7982)

show and link to a wide variety of reflecting telescope optical designs that include negative focal length mirrors and off-axis deflection. (Check them out, but two examples are shown below).

As for a proper, classical Newtonian with its truly 45° diagonal, **it will certainly have been tried by some amateur, somewhere, sometime, if for no other reason than they didn't have access to a primary mirror (blank, or figured) with a hole in the center.**

**Another reason would be viewing convenience** - that side-viewing option is pretty handy when your DIY portable telescope is big and heavy and for mechanical reasons a tall mount high enough to make bottom viewing with an external diagonal (inverting or fancy non-inverting) a pain.

Just for example, LOptics.com's [Building an open-tubed 12.5" F/12.5 Classical Cassegrain By Michael E. Lockwood](http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html)

[![LOptics.com's ](https://i.sstatic.net/bGVIh.jpg)](https://i.sstatic.net/bGVIh.jpg)

[![LOptics.com's ](https://i.sstatic.net/xsgGk.jpg)](https://i.sstatic.net/xsgGk.jpg)

---

---

from <https://astronomy.stackexchange.com/a/49247/7982>

> From [Design and development of a freeform active mirror for an astronomy application](http://dx.doi.org/10.1117/1.OE.53.3.031311) (also [here](https://www.researchgate.net/publication/259645665_Design_and_development_of_a_freeform_active_mirror_for_an_astronomy_application)):
>
> [![](https://i.sstatic.net/Bs0KT.png)](https://i.sstatic.net/Bs0KT.png)

---

From <https://astronomy.stackexchange.com/a/43363/7982>

[![enter image description here](https://i.sstatic.net/gKt9l.png)](https://i.sstatic.net/gKt9l.png)

---

from <https://astronomy.stackexchange.com/a/43977/7982>

[![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**: It looks like. But this is closer to the Cassegrain reflecting telescope with an additional diagonal mirror. The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror.  
> **username_2**: @username_0 well "like a Newtonian" is not a very well-defined term. You haven't laid out your criteria in the question so answers have to judge for themselves what "like a Newtonian" does and does not mean. It would have been better if your comment "The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror" was in the original question if that's what you were thinking.  

---

