<p>That won't work because the shape will be too distorted to be useful. For astronomical purposes, a good quality mirror is needed since the objects you look at are dim and small. If you want a usable astronomical telescope then the optics need to be high quality</p>

<p>If you want to illustrate the properties of telescopes, a simple refracting telescope can be built using the lens from (low power) reading glasses as the objective and a (high power) hand lens as the eyepiece. I've also tried with a shaving mirror, but found the distortion to be too great to form a useful image.</p>

CC BY-SA 3.0
2017-08-15T06:40:48.543Z
2017-08-15T06:40:48.543Z
James K
<p>Sounds kinda crazy and cheap but has anyone ever tried to take a flexible (plastic) mirror and attach a string to the back side then pull the string so it makes a concave surface? I realize it wouldn't be the most accurate device in the world, but if I wanted to build a telescope with a child I think it would be a cheap and fun way to make it. Thoughts? </p>

CC BY-SA 3.0
2017-08-15T06:57:20.547Z
Maybe [something like this](http://www.nymetroparents.com/article/diy-telescope-from-national-geographic-kids-get-outside-guide#.VKB92v8SAA) ?  Won't be a  great telescope but it's quick and might spark interest,
StephenG - Help Ukraine
CC BY-SA 3.0
2017-08-15T13:13:42.460Z
You really talking about the objective, or about the primary?
Carl Witthoft
CC BY-SA 3.0
2017-08-15T13:15:28.227Z
You might be interested (not for building one, just for information) in reading about how active-control large aperture  deformable mirrors are designed.  But typically they are placed in a relay location so the base shape can be flat.  much easier to manufacture.
Carl Witthoft
CC BY-SA 3.0
2017-08-15T15:12:25.997Z
Aluminumized mylar sheeting stretched over a round pipe, be it drum or trashcan, then stretched using a vacuum cleaner on the pipe, can give you a pretty nice reflector. Here's an example: https://www.youtube.com/watch?v=WB3saBxu9Ak People tried them for telescope mirrors back in the 70's, but that info is likely lost in the pre-internet abyss.
Wayfaring Stranger
CC BY-SA 3.0
2017-08-15T18:51:00.340Z
@WayfaringStranger The mylar thing is fine for making little solar ovens, e.g. to cook a hotdog using solar power, but it's nowhere near the quality required for actual telescopes.
Florin Andrei
CC BY-SA 3.0
2017-08-15T18:54:24.973Z
@Shane Wouldn't work. But if you want to show the kids how a telescope is made, try the Galileoscope. It's not very expensive, you could build it in an afternoon, and it's actually usable. It will show you mountains and craters on the Moon, and (just barely) the ring of Saturn. Try to avoid using it handheld because the image is shaking around too much - google "how to make a galileoscope mount", you can make a mount from a cardboard box or an old broomstick.
Florin Andrei
<telescope><telescope-lens>
Possible to make a cheap reflective objective lens with a mirror?
Shane R.
# Possible to make a cheap reflective objective lens with a mirror?

(Asked by: username_0 on 2017-08-15)

Sounds kinda crazy and cheap but has anyone ever tried to take a flexible (plastic) mirror and attach a string to the back side then pull the string so it makes a concave surface? I realize it wouldn't be the most accurate device in the world, but if I wanted to build a telescope with a child I think it would be a cheap and fun way to make it. Thoughts?

**Tags:** telescope, telescope-lens

> **Comments:**
> **username_1**: Maybe [something like this](http://www.nymetroparents.com/article/diy-telescope-from-national-geographic-kids-get-outside-guide#.VKB92v8SAA) ?  Won't be a  great telescope but it's quick and might spark interest,  
> **username_2**: You really talking about the objective, or about the primary?  
> **username_2**: You might be interested (not for building one, just for information) in reading about how active-control large aperture  deformable mirrors are designed.  But typically they are placed in a relay location so the base shape can be flat.  much easier to manufacture.  
> **username_3**: Aluminumized mylar sheeting stretched over a round pipe, be it drum or trashcan, then stretched using a vacuum cleaner on the pipe, can give you a pretty nice reflector. Here's an example: https://www.youtube.com/watch?v=WB3saBxu9Ak People tried them for telescope mirrors back in the 70's, but that info is likely lost in the pre-internet abyss.  
> **username_4**: @username_3 The mylar thing is fine for making little solar ovens, e.g. to cook a hotdog using solar power, but it's nowhere near the quality required for actual telescopes.  
> **username_4**: @username_0 Wouldn't work. But if you want to show the kids how a telescope is made, try the Galileoscope. It's not very expensive, you could build it in an afternoon, and it's actually usable. It will show you mountains and craters on the Moon, and (just barely) the ring of Saturn. Try to avoid using it handheld because the image is shaking around too much - google "how to make a galileoscope mount", you can make a mount from a cardboard box or an old broomstick.  

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## 1 answers

### Answer by username_5 on 2017-08-15. Score: 1

That won't work because the shape will be too distorted to be useful. For astronomical purposes, a good quality mirror is needed since the objects you look at are dim and small. If you want a usable astronomical telescope then the optics need to be high quality

If you want to illustrate the properties of telescopes, a simple refracting telescope can be built using the lens from (low power) reading glasses as the objective and a (high power) hand lens as the eyepiece. I've also tried with a shaving mirror, but found the distortion to be too great to form a useful image.


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