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unknown doublet for making a telescope

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#1 meade4ever

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Posted 17 September 2019 - 01:46 PM

I would like to know if you can use any doublet (here a 100mm f5) to use as an objective for a refractor used at low power (say 20x or below) ?

I don't know anything on this doublet except that it is coated and used for photo application (enlarger ?).



#2 howardcano

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Posted 17 September 2019 - 04:11 PM

Doublets used for objective lenses are designed for the object to be at an infinite distance.  Copier lenses and projection lenses (and enlarger lenses, I assume) are designed for an object that is very close, so they don't work particularly well for a telescope.

 

If you already have the lens, get a cheap cardboard tube and try it!


Edited by howardcano, 17 September 2019 - 04:12 PM.

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#3 dan_h

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Posted 17 September 2019 - 04:11 PM

It may be possible to use the lens for a telescope but then again, it may not.   Not every doublet is meant to be used as a stand alone lens. Many are meant to be used as a part of a more complex lens assembly and the individual elements may not be corrected for color or spherical abberation etc.   Enlargers, projectors, copiers etc all use lens in various combinations to achieve what they need to do.  You need to test the lens you are considering to see if it is useable for a telescope objective. 

 

dan



#4 meade4ever

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Posted 17 September 2019 - 10:31 PM

Thanks a lot for your inputs



#5 williew

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Posted 18 September 2019 - 05:32 AM

A crude test. Hold the lens up in front of you and look at it thru maybe a 25mm eye piece. Move it in or out to bring it to focus and see what you've got.


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