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4.5" American Optical lens

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#1 StarmanDan

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Posted 02 December 2012 - 02:25 PM

I have a 4.5" lens apparently made by American Optical. I'm assuming the 22 inches is the focal length. I've hand held an eyepiece up to it and this appears to be accurate. The lens is quite heavy and in it's own cell. The cell is threaded on the inside on one end but there is a single rivet that prevents using it without drilling out the rivet. I don't remember how I acquired this or what type of lens it is or what it's purpose was in it's past life but I figured it would make a nice lens for a rich field refractor. Has anyone used this type of lens before and how good is it? I've never built a refractor before and am unsure where to start. I figured the easiest thing to do was to gently drill out the rivet and seeing if I can find a machine shop to fabricate a tube of the right length with the correct threads that would match the lens cell. But that could get quite pricy. I really don't want to have to disassemble the lens to try to make my own cell. Any suggestions on where to start? Pics of the lens below:

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#2 StarmanDan

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Posted 02 December 2012 - 02:33 PM

And another:

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#3 orlyandico

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Posted 02 December 2012 - 07:17 PM

Looks like a projection lens of some sort...

Threading a pipe could be costly. Why not find a sonotube of the right diameter or a rolled hastings pipe and put that it? You could friction fit it or drill and tap a few holes in the lens cell.

#4 Conrad1

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Posted 02 December 2012 - 07:49 PM

Before spending any big bucks I'd make a low cost telescope and check it out. F# 4.5 is fast, for a non astronomical design it will likely be imperfect, but it may give cool wide field views. Mock up something low cost and see what you have got!
Ideas:
Consider some kind of plastic pipe fittings for a tube. Both tube and the fittings can be used. Example a sched 40 PVC pipe (nom 4") has an OD of 4.5 inch. A coupling will have a 4.5 inch ID and fit over a 4 inch "pipe". A pipe end cap with a 1.25 inch hole drilled could be the push-pull eyepiece holder.

How much color do you get looking thru it? How big are the stars?

The above is a thought that likely costs (significantly) less than $50.

Conrad

#5 Mike I. Jones

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Posted 02 December 2012 - 08:26 PM

Agree on testing before investing! Most lenses for photographic or enlargement don't make very good astronomical objectives, as they are not corrected to give high-Strehl visual observing. Not saying yours isn't suitable, but be sure you test it first.
Mike

#6 GlennLeDrew

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Posted 03 December 2012 - 01:53 PM

It indeed appears to be a projection lens, probably having three widely spaced elements. If assigned for projection, it will not be corrected for use at infinity and so is expected to suffer spherical aberration--for anything more than the lowest useable powers (5-7mm exit pupils) at any rate.

Make up a zero-cost test rig to assess performance first! For completeness sake, try the lens in both orientations. See how high a power you can achieve before image 'softness' becomes objectionable. Beware daytime testing, where your eye's pupil will act as an aperture stop when it's smaller than the exit pupil.






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