<p>A 50mm aperture with 1000mm focal length is an f/20 ratio - which is a very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vignetting (that's what you see with the 'small image'). With those pre-conditions you won't get a bright image with a wide FOV.</p>

CC BY-SA 4.0
2020-08-28T08:56:58.443Z
2020-08-28T08:56:58.443Z
James K
<p>I am using a convex lens with <span class="math-container">$f=5$</span> cm and <span class="math-container">$5$</span> cm aperture, a concavo-convex lens with <span class="math-container">$f=100$</span> cm and <span class="math-container">$5.5$</span> cm aperture. I am trying to make telescope and it should have magnification of 20 x. But the image coming of eyepiece is very small and cant be seen clearly. Please help with the problem?</p>

CC BY-SA 4.0
2020-08-27T09:59:28.890Z
50mm aperture with 1000mm focal length is a f/20 ration - which is very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vigneting (that's what you see with the 'small image'). With those pre-condition you won't get a bright telescope with a wide FOV.
planetmaker
CC BY-SA 4.0
2020-08-27T19:57:22.927Z
@planetmaker I think that answers the OP's question, perhaps with minimal editing it could be made into a proper answer.
James K
CC BY-SA 4.0
2020-08-28T13:54:19.177Z
What the comments and answer suggest is that you need to read up on things like "exit aperture" and "Field of view" , as those parameters matter -- and will affect your choice of focal lengths for the two main elements in your design.   You might consider going to a 3-or-4 element system if you have some ray-trace software available to assist.
Carl Witthoft
<observational-astronomy><telescope><amateur-observing><telescope-lens>
Problem with making telescope
Siddharth Jain
# Problem with making telescope

(Asked by: username_0 on 2020-08-27)

I am using a convex lens with $f=5$ cm and $5$ cm aperture, a concavo-convex lens with $f=100$ cm and $5.5$ cm aperture. I am trying to make telescope and it should have magnification of 20 x. But the image coming of eyepiece is very small and cant be seen clearly. Please help with the problem?

**Tags:** observational-astronomy, telescope, amateur-observing, telescope-lens
> **Comments:**
> **username_1**: 50mm aperture with 1000mm focal length is a f/20 ration - which is very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vigneting (that's what you see with the 'small image'). With those pre-condition you won't get a bright telescope with a wide FOV.  
> **username_2**: @planetmaker I think that answers the OP's question, perhaps with minimal editing it could be made into a proper answer.  
> **username_3**: What the comments and answer suggest is that you need to read up on things like "exit aperture" and "Field of view" , as those parameters matter -- and will affect your choice of focal lengths for the two main elements in your design.   You might consider going to a 3-or-4 element system if you have some ray-trace software available to assist.  

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## 1 answers

### Answer by username_2 on 2020-08-28. Score: 1

A 50mm aperture with 1000mm focal length is an f/20 ratio - which is a very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vignetting (that's what you see with the 'small image'). With those pre-conditions you won't get a bright image with a wide FOV.


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