Fresnel lenses for a makeshift Galilei telescope? For a kindergarden (pirate) party, friends of me want to build together with the 4 year old kids a functional pirate's telescope for each of the kids. On the web I found full constuction sets, e.g. http://www.astromediashop.co.uk/Telescopes.html (I am not affiliated), but that is only half the fun than improvising and building with household things.
Does anybody has experience building a makeshift telescope using the cardboard tube from a paper kitchen roll, in particular which lenses to use and how to fix them?
The rolls are e.g. 25.5cm long and have an approximate diameter of 4cm. Ideally, we would need two fresnel lenses, I guess, but I am not sure which ones to choose, how to fix them nicely to the tube etc.
Looking forward to your ideas.
Edit: In the meantime, I also found View the Stars Through a DIY Telescope which just states:

[You neeed]: 2 convex lenses (you can get these from a pair of magnifying glasses or by ordering lenses online)

Finding suitable lenses (with right focal length, diameter, weight) is actually the hard part.
Thinking further how I recycle things from home, I got the rough idea to build water lenses but this is clearly out of scope of a kindergarden project, or is it?
My first telescope was made from the lens and eyepiece of an old discarded pair of binoculars. I used a paper towel roll cardboard tube with the objective taped on the front and a flat, circular piece of card stock (with radial cuts near the edge for folding) taped over the other end. It had a hole big enough for the eyepiece to fit, and pieces of tape slightly covering one edge of the hole for friction.
Sliding the eyepiece in and out slightly against the friction provided focus.
It's exciting to see the world big and upside down.
I'd go ahead and try the fresnel lenses for both objective and eyepiece if you can find a pair with different focal lengths, but I think you can also find short focal length lenses in novelty, &quot;dollar&quot;, toy, or other random-object store as pocket tools or toys, or in hobby or tool stores as cheap imitation jewler, gemologist or stamp-collector's loupes.
For a short focal length lens you can even try a drop of water stuck on a glass slide, or a transparent material that hardens transparent.
Or a spherical transparent marble or bead: Ball lens calculator, Ball lens.
If you'd like to &quot;go small&quot; afterwards and explore the microcosm, see foldscope.com, teamfoldscope, Foldscope for more ideas how low-cost projects can be fun and educational.