From: "Ellen Mackenzie" 
To: 
Reply-To: "Ellen Mackenzie" 


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I picked up a batch of surplus Heligon lenses mostly for the glass in them.
I discovered a few of them made excellent RFT eyepieces. In particular, the
90mm and the 50mm. Surprisingly, the stated focal length has little to do
with the actual working focal length, with both lenses having an apparent
working distance of about 25 mm. I have searched the web to find out about
construction details of these lenses but was unable to find much useful
information.
    Does anyone out there have patent numbers, glass types etc.

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I =
picked up a batch=20
of surplus Heligon lenses mostly for the glass in them. I discovered a = few of=20
them made excellent RFT eyepieces. In particular, the 90mm and the 50mm. =

Surprisingly, the stated focal length has little to do with the actual = working=20
focal length, with both lenses having an apparent working distance of = about 25=20
mm. I have searched the web to find out about construction details
= of these=20
lenses but was unable to find much useful =
information.
   =20
Does anyone out there have patent numbers, glass types=20
etc.

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