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ProjectionTV lens as refractor?

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#1 snowcrow

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Posted 17 January 2013 - 07:04 AM

Some years back I picked up a projection TV lens. It has a focal length of 22" and 4" of clear aperture, making it a f/5.5. The cell contains 4 lenses in 3 groups (2;1;1) with baffles between each group.

I built a make-shift telescope using the cell, a piece of 4" PVC sewer pipe, a cap, and a 2" dob focuser I has kicking around.(picture below) Visually it work great with a 20mm plossl EP, but I havn't tried it yet for AP.

Has anyone here tried this before, and is it worth pursuing?

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#2 ed_turco

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Posted 17 January 2013 - 11:33 AM

This is better than one of those plastic, (yes, plastic) multi-element TV lenses from the 1980s. What you have is a real glass Besseler opaque projection lens from years ago.

I've tested these and the correction really isn't so hot.

But, at 20x or a little more, you have a dandy RFT.

#3 snowcrow

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Posted 17 January 2013 - 11:45 AM

I had been looking for another one to maybe make a pair of binoculars.

#4 Darren Drake

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Posted 17 January 2013 - 02:06 PM

I have an old rear projection TV and always wondered about those big lenses. I may try this myself sometime.

#5 Jon Isaacs

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Posted 17 January 2013 - 03:38 PM

I have an old rear projection TV and always wondered about those big lenses. I may try this myself sometime.


As many of you probably know, before TeleVue made eyepieces and telescopes, they made projection lenses for large projection screen televisions.

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#6 snowcrow

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Posted 17 January 2013 - 04:04 PM

Well...that is a bit of trivia I didn't know! The name "TeleVue" kind of gives it away too!

Well if it clear up tonight, I'm going to try an image or two through my new contraption with my EOS 550D. Wish me luck. :grin:

#7 ed_turco

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Posted 17 January 2013 - 08:02 PM

The rear projection lenses from the 1980s were made of plastic and had its object plane very close to the vidicon.
Later lenses from computer projection units also had this feature, but were made of glass. The correction for telescope use was poor, and it would very difficult to get the object, now actually the image plane and the eyepiece focal plane to coincide.

Opaque projection units are the best of the bunch, but really not that good.


Ed






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