<p>The images in the question show a progression from severely out of focus (large disc with shadow of secondary mirror) to nearly in focus (six-way diffraction pattern). There are a few things you can try:</p>

<ul>
<li>In daylight, make sure that the the secondary mirror support vanes are parallel to the optical tube axis. This may reduce the diffraction artifacts.</li>
<li>The light from a very bright source will show weaknesses in all but the finest optical systems. Use a neutral-density "moon" filter to reduce the overall brightness.</li>
<li>Point sources make it easier to find the point of best focus. Focus on a nearby star faintly visible to the naked eye, then aim the telescope at the planet and wait for steady air.</li>
</ul>

CC BY-SA 3.0
2017-01-22T19:25:32.063Z
Thanks, I will try that as soon as I will be able to. Currently there are too many clouds in the sky. :-(
palsch
CC BY-SA 3.0
2017-01-23T13:35:48.680Z
This does not address the obvious massive focus error in the images posted.
Carl Witthoft
CC BY-SA 3.0
2017-01-23T17:05:08.033Z
@CarlWitthoft Updated.
Mike G
CC BY-SA 3.0
2017-01-22T19:23:57.123Z
2017-01-23T17:04:46.743Z
2017-01-23T17:04:46.743Z
Mike G
<p>I think you just need to keep winding the focusser knob, you are way past focus in the first picture but you can see the image get smaller as you approach focus. The last image looks like you may be there or thereabouts, I think the colored diffraction patterns are caused by the camera optics.</p>

CC BY-SA 3.0
2017-01-22T11:47:08.527Z
I see the colored diffraction patterns with my eye as well.
palsch
CC BY-SA 3.0
2017-01-21T00:21:19.133Z
2017-01-21T00:21:19.133Z
Dave
<p>Today I try to watch Venus through a mirror telescope. I am no professional. No matter what lens I take, I see this image:<a href="https://i.sstatic.net/N3ZeI.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/N3ZeI.png" alt="strange"></a> What am I doing wrong?</p>

<p>Edit: What happens when I refocus:<a href="https://i.sstatic.net/ZPmGu.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZPmGu.png" alt="focus 1"></a><a href="https://i.sstatic.net/ZXYC1.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZXYC1.png" alt="focus 2"></a><a href="https://i.sstatic.net/yPnXL.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/yPnXL.jpg" alt="focus 3"></a>Sorry for the bad quality, I didn't manage to make better images. :-(</p>

CC BY-SA 3.0
2017-01-20T18:13:12.050Z
That looks like the shadow of the spider for the 2° mirror on an illuminated primary. Your focus would have to be **way** off to see that.
Wayfaring Stranger
CC BY-SA 3.0
2017-01-20T18:30:45.257Z
I second that, this looks like your lens is out of focus. Try to play around with the screws you have on the scope.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-01-20T19:04:24.110Z
@AtmosphericPrisonEscape I did that, and the results don't quite match what I expect it to be. (edit) Maybe I'm expecting too much? (Changing the lens also doesn't work, I tried all I have.)
palsch
CC BY-SA 3.0
2017-01-20T19:23:06.140Z
Looks like you're on the right track towards you last image. Can you continue focusing past that point? It might also help to identify what telescope/hardware you're using.
zephyr
CC BY-SA 3.0
2017-01-20T19:27:41.053Z
@zephyr When I focus past that point, the image is getting bigger and bigger again, until the first image is reached. Writing on the telescope is "tasco galaxsee".
palsch
CC BY-SA 3.0
2017-01-20T19:35:34.620Z
The manual for those telescopes indicates there are two ways to focus. A "focus knob" and a "focus tube". Clearly one of these is waaay off as Wayfaring Stranger indicated. Somehow your telescope got really out of whack. You'll need to likely play with both of these two focusing objects.
zephyr
CC BY-SA 3.0
2017-01-20T19:45:13.923Z
@zephyr I can't find anything like that in my manual: http://imgur.com/uxY8KVZ (telescope number is 46-114500)
palsch
CC BY-SA 3.0
2017-01-20T19:49:22.997Z
See the second image in the manual. Both C and D in this image are focusing tools. Unfortunately the manual is not very verbose about what these are and how to use them and I'm not sure anyone here could give more specific information without seeing your telescope in person or being intimately familiar with it. You might get better help by calling the company's help line if you can (or emailing them).
zephyr
CC BY-SA 3.0
2017-01-20T19:56:28.137Z
Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/52156/discussion-between-palsch-and-zephyr).
palsch
CC BY-SA 3.0
2017-01-24T16:29:25.227Z
Try focusing on something on the ground, preferably far away (if you have a view of the horizon that would be ideal).  Venus is a point of light to the naked eye, and it can be difficult to tell when you've focused on it properly. If you're aiming at, say, a distant tree, mountain, or building, it's going to be easier to see a blurry but recognizable image of the target when it's out of focus, and to adjust the focusing knob until it's clear. That will give you at least a rough idea of where you need to be to focus on infinity. *Then* point your telescope at the sky.
Keith Thompson
<telescope>
Mirror telescope: blocked sight
palsch
# Mirror telescope: blocked sight

(Asked by: username_0 on 2017-01-20)

Today I try to watch Venus through a mirror telescope. I am no professional. No matter what lens I take, I see this image:[![strange](https://i.sstatic.net/N3ZeI.png)](https://i.sstatic.net/N3ZeI.png) What am I doing wrong?

Edit: What happens when I refocus:[![focus 1](https://i.sstatic.net/ZPmGu.png)](https://i.sstatic.net/ZPmGu.png)[![focus 2](https://i.sstatic.net/ZXYC1.png)](https://i.sstatic.net/ZXYC1.png)[![focus 3](https://i.sstatic.net/yPnXL.jpg)](https://i.sstatic.net/yPnXL.jpg)Sorry for the bad quality, I didn't manage to make better images. :-(

**Tags:** telescope
> **Comments:**
> **username_1**: That looks like the shadow of the spider for the 2° mirror on an illuminated primary. Your focus would have to be **way** off to see that.  
> **username_2**: I second that, this looks like your lens is out of focus. Try to play around with the screws you have on the scope.  
> **username_0**: @AtmosphericPrisonEscape I did that, and the results don't quite match what I expect it to be. (edit) Maybe I'm expecting too much? (Changing the lens also doesn't work, I tried all I have.)  
> **username_3**: Looks like you're on the right track towards you last image. Can you continue focusing past that point? It might also help to identify what telescope/hardware you're using.  
> **username_0**: @zephyr When I focus past that point, the image is getting bigger and bigger again, until the first image is reached. Writing on the telescope is "tasco galaxsee".  
> **username_3**: The manual for those telescopes indicates there are two ways to focus. A "focus knob" and a "focus tube". Clearly one of these is waaay off as Wayfaring Stranger indicated. Somehow your telescope got really out of whack. You'll need to likely play with both of these two focusing objects.  
> **username_0**: @zephyr I can't find anything like that in my manual: http://imgur.com/uxY8KVZ (telescope number is 46-114500)  
> **username_3**: See the second image in the manual. Both C and D in this image are focusing tools. Unfortunately the manual is not very verbose about what these are and how to use them and I'm not sure anyone here could give more specific information without seeing your telescope in person or being intimately familiar with it. You might get better help by calling the company's help line if you can (or emailing them).  
> **username_0**: Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/52156/discussion-between-palsch-and-zephyr).  
> **username_4**: Try focusing on something on the ground, preferably far away (if you have a view of the horizon that would be ideal).  Venus is a point of light to the naked eye, and it can be difficult to tell when you've focused on it properly. If you're aiming at, say, a distant tree, mountain, or building, it's going to be easier to see a blurry but recognizable image of the target when it's out of focus, and to adjust the focusing knob until it's clear. That will give you at least a rough idea of where you need to be to focus on infinity. *Then* point your telescope at the sky.  

---

## 2 answers

### Answer by username_5 on 2017-01-22. Score: 2

The images in the question show a progression from severely out of focus (large disc with shadow of secondary mirror) to nearly in focus (six-way diffraction pattern). There are a few things you can try:

* In daylight, make sure that the the secondary mirror support vanes are parallel to the optical tube axis. This may reduce the diffraction artifacts.
* The light from a very bright source will show weaknesses in all but the finest optical systems. Use a neutral-density "moon" filter to reduce the overall brightness.
* Point sources make it easier to find the point of best focus. Focus on a nearby star faintly visible to the naked eye, then aim the telescope at the planet and wait for steady air.

> **Comments:**
> **username_0**: Thanks, I will try that as soon as I will be able to. Currently there are too many clouds in the sky. :-(  
> **username_6**: This does not address the obvious massive focus error in the images posted.  
> **username_5**: @CarlWitthoft Updated.  

---

### Answer by username_7 on 2017-01-21. Score: 2

I think you just need to keep winding the focusser knob, you are way past focus in the first picture but you can see the image get smaller as you approach focus. The last image looks like you may be there or thereabouts, I think the colored diffraction patterns are caused by the camera optics.

> **Comments:**
> **username_0**: I see the colored diffraction patterns with my eye as well.  

---

