Project : HNU-75 - Moon watcher

[Chriske]

Every year in our local observatory Urania (Hove - Belgium) new courses starts. One of them is a 'basic astronomy' course for 12 to 15 year old. Most of these kids have no telescope(yet). So I thought why not build these kids' at the end of that course a very basic scope, say kind of a 'Moon-watcher'. If they're really interested in astronomy they can always buy a larger scope later on.
They idea is to build these kids a very basic scope each and sell it at a very low price.
Busy designing this HNU-75 this very moment. HNU-75 stands for Hershel-Newtonian-Urania, its primary mirrors diameter is 75mm(3").
Urania is rather obvious, Herschel : because I didn't  want a secondary mirror in the lightpad, and Newtonian because there is actually a secondary mirror involved after all, but that secondary mirror is located at the opposite side of the focuser but outside the 'tube'. So during observing you have the 'Newton telescope feel' but no obstruction. Focal length 1400mm(55"), so f/18. Why that rather long FL, well I ran a Hershel-system through WinSpot(basic Raytrace) and the only acceptable combination to end up with a diffraction limited Herschelian was f/18. Advantage of this longer telescope is a somewhat larger primary image, so ideal for Moon, planets and also globular clusters.
The last 35 years I've organized telescope making courses at our observatory.  Every year new members made telescopes up to 300mm(12"). At the end of each course I was left with a pile of tools, nobody wanted to take it's tool at home. So I have literal tons of glass disks waiting to be used, collecting dust. The idea is to reuse these disk and make primary mirrors out of them for these little scopes. Out of 1 single 10" mirror I can core out seven 3" mirrors. Saves me lots of work. Aluminizing and coating is no problem at all because I have my own coater(up to 24").
Our observatory will pay half of the price of these scopes. So the pupils parents are left with (about) 25€ to pay for their scope. I think that's a fair price for this 'telescope on a stick'. Included are 2 Kelner eyepieces, dob mount, laser to collimate the scope and a LED-finderscope.
Almost all parts will be 3D-printed, saves me LOTS of work(again).

This is how these scopes will look like. Focuser and Dob-mount still to be drawn.

 

[Gina]

That's an interesting design

[furrysocks2]

Cool!

Very small exit pupils... 25mm EP (56x) gives 1.34mm, 10mm EP (140x) gives 0.53mm. I guess kids can tolerate that?

 

[Stu]

1 hour ago, furrysocks2 said:

Cool!

Very small exit pupils... 25mm EP (56x) gives 1.34mm, 10mm EP (140x) gives 0.53mm. I guess kids can tolerate that?

 

An old man like me can tolerate that, so I'm sure the kids will be fine ?

Great fun project, and you are doing an excellent job getting them interested at an early age. Look forward to seeing the final design and end 'product' 

[furrysocks2]

26 minutes ago, Stu said:

An old man like me can tolerate that, so I'm sure the kids will be fine ?

[Chriske]

First thing to do is flatten all these old tools.
I made myself a little wooden device containing an angle-grinder with a diamond disk. It takes me just a few minutes to flatten one tool.

https://youtu.be/atqwnxBUOe8

[Chriske]

Next step : hogging out three 10" mirrors. Each mirror's back serves as tool for another mirror. When all mirrors are done I have no tool left, so no waste of material.
Doing so I will end up with three meniscus shaped mirrors.