<p>In a Newtonian reflector, as pictured, the secondary mirror does block some of the light, but maybe less than you think.  Even if the secondary were half the diameter of the primary, it would only block 1/4 of the light ($\ (1/2)^2$).  In a more typical case the secondary would be somewhat smaller - perhaps a quarter of the size of the primary (or less).  Hence a $1/16th$ or less is blocked, which is not too bad.</p>

CC BY-SA 3.0
2016-12-10T14:23:19.290Z
2016-12-10T14:23:19.290Z
Dr Chuck
<p>When I'm looking at the inside of a mirror telescope:</p>

<p><a href="https://i.sstatic.net/U76M9.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/U76M9.png" alt="enter image description here"></a></p>

<p>I'm wondering why the secondary mirror does not block half of the incoming light? Is it "transparent" in this direction?</p>

CC BY-SA 3.0
2016-12-10T17:18:16.827Z
Why in the world would you think it **doesn't** block incoming light?
Carl Witthoft
CC BY-SA 3.0
2016-12-10T18:49:22.393Z
@CarlWitthoft My guess is that netik thinks there should be a "hole" in your image where the secondary mirror is. This is nothing more than a basic misunderstanding with how the optics of a telescope works.
zephyr
CC BY-SA 3.0
2016-12-11T13:04:50.327Z
@zephyr you're probably right there.  Shall we regal him w/ stories of Fourier Transforms? :-)
Carl Witthoft
CC BY-SA 3.0
2016-12-11T17:33:53.613Z
yeah @Zephyr that's basically what I thought haha.
netik
CC BY-SA 3.0
2016-12-11T18:00:47.790Z
Perfectly reasonable thought unless you really learn how optics works. Unfortunately, its fairly complicated stuff and there are entire textbooks just on optics. It looks like the answer you accepted didn't fully explain why there isn't a dark spot in your image. If you're still interested, I can try to provide a brief answer to explain that.
zephyr
CC BY-SA 3.0
2016-12-11T22:57:41.157Z
You can easily test yourself that you don't necessarily block the centre of the image: Look at something distant (e.g. out the window) with just one eye open, then hold something small in front of that eye (pin head, flat side of small screwdriver or other). You'll notice that as long as you focus something distant, you only get blurring, but the small object won't block part of your field of view.
Alex
<telescope>
Mirror Telescope
netik
# Mirror Telescope

(Asked by: username_0 on 2016-12-10)

When I'm looking at the inside of a mirror telescope:

[![enter image description here](https://i.sstatic.net/U76M9.png)](https://i.sstatic.net/U76M9.png)

I'm wondering why the secondary mirror does not block half of the incoming light? Is it "transparent" in this direction?

**Tags:** telescope
> **Comments:**
> **username_1**: Why in the world would you think it **doesn't** block incoming light?  
> **username_2**: @CarlWitthoft My guess is that netik thinks there should be a "hole" in your image where the secondary mirror is. This is nothing more than a basic misunderstanding with how the optics of a telescope works.  
> **username_1**: @zephyr you're probably right there.  Shall we regal him w/ stories of Fourier Transforms? :-)  
> **username_0**: yeah @Zephyr that's basically what I thought haha.  
> **username_2**: Perfectly reasonable thought unless you really learn how optics works. Unfortunately, its fairly complicated stuff and there are entire textbooks just on optics. It looks like the answer you accepted didn't fully explain why there isn't a dark spot in your image. If you're still interested, I can try to provide a brief answer to explain that.  
> **username_3**: You can easily test yourself that you don't necessarily block the centre of the image: Look at something distant (e.g. out the window) with just one eye open, then hold something small in front of that eye (pin head, flat side of small screwdriver or other). You'll notice that as long as you focus something distant, you only get blurring, but the small object won't block part of your field of view.  

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## 1 answers

### Answer by username_4 on 2016-12-10. Score: 2 | Accepted answer

In a Newtonian reflector, as pictured, the secondary mirror does block some of the light, but maybe less than you think. Even if the secondary were half the diameter of the primary, it would only block 1/4 of the light ($\ (1/2)^2$). In a more typical case the secondary would be somewhat smaller - perhaps a quarter of the size of the primary (or less). Hence a $1/16th$ or less is blocked, which is not too bad.


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