Making a viewfinder from 82mm 2-element achro

[nebula]

Hi everyone, I just bought a 82mm 2-element achromatic. 320mm f.l. Multi-coated lense to use to make a view finder.

Im looking at using 89mm Straight 300mm induction pipe (aluminium) as housing. I really have little idea of what I am doing but figure I can fit the lense in the front of it, then bung a focuser on the other end to make up the other 2cm focal length? and hey presto, somthing tells me it is not this simple?

The lense is from university optics and the pipe from ebay. I am yet to buy the pipe so perhaps there is a more suitable candidate, I am looking for the option with as little cutting as possible. Also a cheap focuser suggestion would be great.:help:

[astro744]

Your focuser will likely be longer than 2cm so your tube length will be shorter than 30cm but it depends on the type of focuser you had in mind. I made a 50mm finder from and old binocular lens and some plumbing pipe. I used some threaded plumbing reducers to step down from the 50mm pipe to a diameter suitable for holding the eyepiece which was the eye lens from the binocular. (The binocular prism was damaged, that's why I had the lenses spare). The focussing is done by turning the helical thread on the reducer. Even with this setup the focuser is approx 5cm.

Be prepared for some cutting as it's more likely than not. Note if you want a diagonal your pipe is shorter again.

[PhilW]

Here is a cheap focuser:

http://www.surplusshed.com/pages/item/l3630.html

Making a finder is more or less as you describe. You can improve it with matte black paint inside, perhaps a baffle, and a dewshield. Surplus Shed also has cheap diagonals. You can also use a postal mailing tube as a lightweight alternative. It just needs to be painted for waterproofing.

Phil

[nebula]

Thanks for the tips Phil and Astro, love the idea of a getto postal pack viewfinder. That focuser would have been perfect but is out of stock :(

Might have a look at pipe sizes and fittings availalble at bunnings.

[PhilW]

Bunnings has 1m lengths of PVC pipe in 80mm and 90mm diams. That would make a nice tube, albeit heavier than the postal tube idea. I like things to be light to make balancing easier.

You could try a 'wanted' ad in the accessories forum for a spare focuser. I am sure lots of people have old ones lying around. In many cases people have upgraded cheap stock focusers for aftermarket ones. The cheap stock ones are fine for finders. Or maybe cobble together a homemade one with the helical pipe fittings you can find at Bunnings?

Phil