<p>Diffraction has nothing to do with whether you are using a lens or a mirror in your telescope. Diffraction occurs for the same reason in reflecting telescopes as it does in refracting telescopes: <a href="https://www.telescope-optics.net/diffraction.htm" rel="nofollow noreferrer">it is passing through an aperture</a>.  </p>

<p>There is no threshold aperture diameter for which you have diffraction. No matter how big the aperture, you will have diffraction. No matter the shape of the aperture, you will also have diffraction. This happens on any aperture. </p>

<p>However, the bigger the aperture, the smaller the diffraction pattern, or <a href="https://en.wikipedia.org/wiki/Airy_disk" rel="nofollow noreferrer">Airy disc</a>. In 
<a href="http://blog.ulverstonvictoria.cumbria.sch.uk/physics/files/2016/02/Astrophysics-Chapter-1.pdf" rel="nofollow noreferrer">page 19 of the document you mention in the comments</a>, there is a relation between the size of the aperture and the size of the Airy disc. </p>

<p>To clarify what is said in the first paragraph of page 18</p>

<blockquote>
  <p>If the telescope is replaced by one with a narrower objective, the images of the two stars would overlap too much and the observer would not be able to see them as separate stars.</p>
</blockquote>

<p>If the two stars are replaced with two stars that are close enough together, the images of the two stars would overlap too much and in this case the observer would not be able to see them as separate stars either.</p>

<hr>

<p>As a side note, refracting telescopes aren't necessarily smaller than reflecting telescopes. There are <a href="https://en.wikipedia.org/wiki/List_of_largest_optical_refracting_telescopes" rel="nofollow noreferrer">refracting telescopes with a lens more than a meter in diameter</a>.  A lot bigger than most amateur reflecting telescopes ! </p>

CC BY-SA 4.0
2020-04-21T14:53:28.063Z
This makes sense, but the question I was trying to ask was how the mirror acts as an aperture? This is the misconception that's stopping me from understanding everything else
XXb8
CC BY-SA 4.0
2020-04-21T14:58:22.783Z
@XXb8 When light passes through an aperture, it is diffracted. If you have a cardboard tube with no optics in it, light will still be diffracted.
usernumber
CC BY-SA 4.0
2020-04-21T15:04:03.173Z
So how would it diffract on a mirror? Around the edges of the mirror or does light spread after it's reflected?
XXb8
CC BY-SA 4.0
2020-04-22T03:46:12.573Z
It doesn't diffract because of the mirror, it diffracts when it passes through the aperture at the opposite end of the telescope. The mirror has nothing to do with the diffraction.
Jim421616
CC BY-SA 4.0
2020-04-21T14:43:54.623Z
2020-04-21T15:02:24.113Z
2020-04-21T15:02:24.113Z
usernumber
<p>It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.</p>

<p>But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?</p>

CC BY-SA 4.0
2020-04-21T14:28:02.640Z
@usernumber I don't understand how. Looking at reflecting telescope set ups in diagrams, it doesn't look like the mirror could act as an aperture
XXb8
CC BY-SA 4.0
2020-04-21T14:29:43.657Z
If you use a refracting telescope with a lens twice as big, or ten times as big, you will still have diffraction. The diffraction in a refracting telescope is not caused by the size of the lens.
usernumber
CC BY-SA 4.0
2020-04-21T14:32:38.243Z
@usernumber on this link page 18 http://blog.ulverstonvictoria.cumbria.sch.uk/physics/files/2016/02/Astrophysics-Chapter-1.pdf  this is why I'm confused. It implied the lens itself is the aperture that causes diffraction. Is this true? If so, how can a mirror have the same effect? It's not exactly a 'gap'?
XXb8
CC BY-SA 4.0
2020-04-21T14:42:28.633Z
Diffraction happens on apperture, thus the round opening (or however shaped) opening where light passes into the optics - whether it's a mirror or a lens is irrelevant. Don't confuse diffraction with refraction.
planetmaker
CC BY-SA 4.0
2020-04-21T16:41:00.313Z
"Opening" would be better than "gap;" in any case try not to get hung up on word choice. Any objective of finite size will have diffraction inversely related to its size.
Mike G
CC BY-SA 4.0
2020-04-21T17:04:13.507Z
@MikeG So how would a mirror serve as an 'opening'? Would diffraction occur around the edges of the mirror or does diffraction happen after reflection (if so, how?)?
XXb8
CC BY-SA 4.0
2020-04-21T18:34:11.560Z
A mirror is a region of reflection surrounded by non-reflection, just as a lens is a region of transmittance surrounded by non-transmittance.
Mike G
CC BY-SA 4.0
2020-04-21T18:37:35.593Z
A mirror simply "folds" the beam path.  Since a reflector 'scope folds only that portion of the beam which hits the mirror,  it's functionally identical to a lens of that size so far as the aperture goes. From your many comments here, it's clear you could benefit from reading basic tutorials on how optics work.  Please do so.
Carl Witthoft
CC BY-SA 4.0
2020-04-21T18:38:22.583Z
I’m voting to close this question because its premise is false.
Carl Witthoft
CC BY-SA 4.0
2020-04-21T19:05:08.467Z
A mirror is exactly an aperture.
ProfRob
<observational-astronomy><telescope><amateur-observing><newtonian-telescope>
How does diffraction occur in a reflecting telescope?
XXb8
# How does diffraction occur in a reflecting telescope?

(Asked by: username_0 on 2020-04-21)

It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.

But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?

**Tags:** observational-astronomy, telescope, amateur-observing, newtonian-telescope
> **Comments:**
> **username_0**: @usernumber I don't understand how. Looking at reflecting telescope set ups in diagrams, it doesn't look like the mirror could act as an aperture  
> **username_1**: If you use a refracting telescope with a lens twice as big, or ten times as big, you will still have diffraction. The diffraction in a refracting telescope is not caused by the size of the lens.  
> **username_0**: @usernumber on this link page 18 http://blog.ulverstonvictoria.cumbria.sch.uk/physics/files/2016/02/Astrophysics-Chapter-1.pdf  this is why I'm confused. It implied the lens itself is the aperture that causes diffraction. Is this true? If so, how can a mirror have the same effect? It's not exactly a 'gap'?  
> **username_2**: Diffraction happens on apperture, thus the round opening (or however shaped) opening where light passes into the optics - whether it's a mirror or a lens is irrelevant. Don't confuse diffraction with refraction.  
> **username_3**: "Opening" would be better than "gap;" in any case try not to get hung up on word choice. Any objective of finite size will have diffraction inversely related to its size.  
> **username_0**: @MikeG So how would a mirror serve as an 'opening'? Would diffraction occur around the edges of the mirror or does diffraction happen after reflection (if so, how?)?  
> **username_3**: A mirror is a region of reflection surrounded by non-reflection, just as a lens is a region of transmittance surrounded by non-transmittance.  
> **username_4**: A mirror simply "folds" the beam path.  Since a reflector 'scope folds only that portion of the beam which hits the mirror,  it's functionally identical to a lens of that size so far as the aperture goes. From your many comments here, it's clear you could benefit from reading basic tutorials on how optics work.  Please do so.  
> **username_4**: I’m voting to close this question because its premise is false.  
> **username_5**: A mirror is exactly an aperture.  

---

## 1 answers

### Answer by username_1 on 2020-04-21. Score: 3

Diffraction has nothing to do with whether you are using a lens or a mirror in your telescope. Diffraction occurs for the same reason in reflecting telescopes as it does in refracting telescopes: [it is passing through an aperture](https://www.telescope-optics.net/diffraction.htm).

There is no threshold aperture diameter for which you have diffraction. No matter how big the aperture, you will have diffraction. No matter the shape of the aperture, you will also have diffraction. This happens on any aperture.

However, the bigger the aperture, the smaller the diffraction pattern, or [Airy disc](https://en.wikipedia.org/wiki/Airy_disk). In
[page 19 of the document you mention in the comments](http://blog.ulverstonvictoria.cumbria.sch.uk/physics/files/2016/02/Astrophysics-Chapter-1.pdf), there is a relation between the size of the aperture and the size of the Airy disc.

To clarify what is said in the first paragraph of page 18

> If the telescope is replaced by one with a narrower objective, the images of the two stars would overlap too much and the observer would not be able to see them as separate stars.

If the two stars are replaced with two stars that are close enough together, the images of the two stars would overlap too much and in this case the observer would not be able to see them as separate stars either.

---

As a side note, refracting telescopes aren't necessarily smaller than reflecting telescopes. There are [refracting telescopes with a lens more than a meter in diameter](https://en.wikipedia.org/wiki/List_of_largest_optical_refracting_telescopes). A lot bigger than most amateur reflecting telescopes !

> **Comments:**
> **username_0**: This makes sense, but the question I was trying to ask was how the mirror acts as an aperture? This is the misconception that's stopping me from understanding everything else  
> **username_1**: @XXb8 When light passes through an aperture, it is diffracted. If you have a cardboard tube with no optics in it, light will still be diffracted.  
> **username_0**: So how would it diffract on a mirror? Around the edges of the mirror or does light spread after it's reflected?  
> **username_6**: It doesn't diffract because of the mirror, it diffracts when it passes through the aperture at the opposite end of the telescope. The mirror has nothing to do with the diffraction.  

---

