I'm going to build Telescop

[Karls48 (Karl)]

Last night I won on eBay 3 elements136mm Lens cell. Focal length 457mm. It was advertised as APO, I don’t know if it is true but from my research on net I know it come from Opaque Projector and it is C/A corrected.
I’m going to build telescope with it. I never attempted to build telescope before so any suggestions are welcome. I’m thinking to build plywood tube and use 2” rack and pinion focuser. Focuser diameter is only 86mm so I will have to find way how to attach it to 136mm tube. Also, how to attach Lens cell to the tube without having correct tread?

[okiscopey (Mike)]

You may be disappointed with the performance of this type of lens.

It sounds like it comes from an episcope (opaque projector) which is designed to project an image of an A4 or similar-sized document or photograpic print onto a screen. You need a lot of light on the doccument and a fast lens (hence the f/3.36) to achieve a decent image brightness for the benefit of an audience.

Using the lens the ‘opposite way’, you will get an image circle of 30cm or so, and magnifying just a small central part of this with an eyepiece will show all the defects of a lens that was designed for a completely different purpose.

The best use of this lens would be to project a wide-field sky view onto a big piece of photographic negative. You could then do your own ‘Palomar sky survey’!

Hopefully someone else will chip in with something more positive!

[Garyh]

Hi Karl, I saw this lens on ebay and realising its a projector lense I stayed clear from it. They just don`t make good telescopes as mentioned by Mike above as used backwards you just won`t get a crisp image. Have tried it with a older lense and the iamges were very soft even at very low magnification..would work great as a large photographic plate camera but!
sorry about not being very positive.
cheers

[Karls48 (Karl)]

Oh well, that was $100 mistake. Now if any one knows where I can get 300mm x 300mm CCD chip for $100 I could salvage this project.
But seriously, I will have to think if I should spent another $150 on 2”diagonal, eyepiece and cheap focuser and see how it will come out. Hmm I need 2”diagonal and eyepiece anyhow.
Thanks for advice guys.

[Glenhuon (Bill)]

I just bought an 80mm x 400 FL lens from Surplus Shed in USA on Ebay. Doesnt look too bad and was pretty cheap ($29US). Its a coated doublet mounted in a plastic housing and I intend to use it for solar viewing. The same outfit have a 80 x 900 FL for the same price. Postage was very reasonable and got here in 8 days.
BTW - if anyone knows of a supplier of thin walled Aluminium or SS tubing in NSW I need some to complete the project.