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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?

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  • $\begingroup$ Maybe something like this ? Won't be a great telescope but it's quick and might spark interest, $\endgroup$ – StephenG Aug 15 '17 at 6:57
  • $\begingroup$ You really talking about the objective, or about the primary? $\endgroup$ – Carl Witthoft Aug 15 '17 at 13:13
  • $\begingroup$ 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. $\endgroup$ – Carl Witthoft Aug 15 '17 at 13:15
  • $\begingroup$ 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: 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. $\endgroup$ – Wayfaring Stranger Aug 15 '17 at 15:12
  • $\begingroup$ @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. $\endgroup$ – Florin Andrei Aug 15 '17 at 18:51
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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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