Plans for 127mm F9.4 folded refractor

[chrisp9au (Chris)]

3 years ago I bought a 127mm F9.4 objective from Surplus Shed in the USA, http://www.surplusshed.com/pages/item/l3967.html
I tried putting it in a tube, but finished up with a very unstable beast, and since then it has sat in the shed!

I'm now playing with the idea of a folded refractor design, to be setup for 'table top' use, on a small Dobsonian base.
My brother in law is very interested in looking through a telescope, but he's a quadriplegic; this design would allow him access, and the rest of the family would probably like it too.
I've managed to source two 100mm optical flats from Surplus Shed.
Folding the light path path introduces just one more surface than a traditional design, and allows for a fixed eyepiece position.
Field rotation?, yes, but I'm only interested in visual, not photography.

I've had a lot of fun drawing up the design in Google Sketchup, which I would recommend to everyone for this type of project.
Google Sketchup can be downloaded from http://sketchup.google.com/intl/en/download/index.html

A screenshot of my design is attached.
The 'box' is 500mm x 380mm x 180mm, the light path is 1200mm.
The Dob mount provides a fixed eyepiece position 400mm above table top height.

I'm interested in comments from anyone who has tackled a project like this, and any pitfalls that I might not be aware of.

Cheers

Chris

[mswhin63 (Malcolm)]

I really like the idea, although one thing will come to mind and that is internal reflections inside. It will need to be very non reflective. I would be interested to know if this concept could be defined for AP as well.

Consider extending tube in some of the light paths instead of holes in the box.

[chrisp9au (Chris)]

Hi Malcom,

The holes in the internal walls are baffles. they will be sized to permit only the light path. All internal surfaces will be lined with black felt from Spotlight.
There should not be any unwanted reflections. and of course there is a lid on the top!

Cheers

Chris

[omegacrux (David)]

Cool idea be great to see the finished scope ( box of Dob )

[Saturnine (Jeff)]

Hi Chris

Sounds like a great project, the Surplus Shed 127 x 1200 mm lens a fine lens, made a conventional scope with one of them a few months ago and for its' price it performs very well. In good seeing I've pushed it to 400X on the planets and double stars.
The design looks quite practical, can't see any real flaws in your design as long as everything is lined up to the optical axis' and the focuser is also centred to the "dob" altitude bearing. Have used black felt from Spotlight for anti reflective material on several scopes, once the felt is glued in place I've given it a light spray with flat black paint to help seal the fibres, seems to prevent any shedding.
Have fun with the project and please give progress reports.

Jeff

[anj026]

Hi Chris

You have a great idea there. Here is a link to a similar concept that might give you some food for thought.
http://www.robertmartinayers.org/SandT-December06.pdf

Regards
Andy