Wooden Mandrel Form for Fiberglass OTA

[theropod]

Hi all,

I am thinking of turning a wooden form upon which to lay down fiberglass mat to make the main tube for a small refractor. I have a nice 60mm objective salvaged from a set of hopelessly broken binoculars. I bought an inexpensive 1 1/4” rack and pinion focuser with a 2” tube attachment point. Since I know the focal length of the objective wouldn’t turning down a closed grain hardwood mandrel/form work for laying down multiple layers of fiberglass? If I smooth out the mandrel, with appropriate tapering from 60mm to 2”, shouldn’t that work? I’m also thinking a good layer of car wax would allow the fiberglass to release after curing. I could make the form in 3 pieces. Imagine two main halves divided by a slat that could be removed and allow the form to collapse. If I made extended pieces on the form where I could place automotive radiator hose clamps to hold everything together I cannot see why it wouldn’t work. Should I think about inserts to strengthen the objective cell mount point l, and the focuser’s?

Should I just go buy some PVC plumbing parts and make that work?

[johninderby]

Aluminum tube is better. It’s what I used when I built my own refractor.

[theropod]

OK, that’s impressive. Now the search for AL tube begins!

[MarkAR]

Sounds like a great project.

[fwm891]

M-Machine have a huge range of tubing they will cut to length as required: http://preview.m-machine.co.uk/CATALOGUES/metals catalogue.pdf

Edited April 6, 2020 by fwm891
text added

[Robert72]

Or carbon? You can get stock carbon that size. Not too sure about price? About £150?

[theropod]

2 hours ago, Robert72 said:

Or carbon? You can get stock carbon that size. Not too sure about price? About £150?

Oh my, that’s WAY over the top for this tiny little scope project. I think the focuser cost me $12.50 off eBay, and the 60mm objective was free as the binoculars were given to me before I drove over them with my pickup (they fell off the bed I had laid them on in an absent minded moment). This is just a make work project, and maybe practice of I find a larger objective lens in the future. I’ll probably give this away in the end, but I want to test it on my DIY alt/az stepper driven mount. I expect the finished weight with EP to be under a pound, and on that mount should really shine.

The FL of the objective is less than 6”, and whatever I settle on won’t be a bank breaker, so a hunk of carbon fiber tube that short might not be prohibitive. I’ll look into it.

I wouldn’t even be bothering with this if the objective was junk, but apparently the majority of the cost of the binoculars was tied up in that front glass. Before removing the lens from its tapered housing I jerry-rigged a PVC adapter to hold my 1 1/4” 12.5mm, and the views were awesome. Getting the prisms out of the path really helped a lot. No CA even on a full moon, and Andromeda just fit in the FOV, and crisp as could be.