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Can a bi-convex lens be replaced with two plano-convex lenses back to back?

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#1 gbambo

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Posted 10 January 2017 - 08:38 PM

It seems to me, that a bi-convex lens could be replaced by two plano-convex lenses, positioned plano-to-plano with zero separation, to the same effect.

 

Is that true? What are the downsides? Should they be glued or the gap oiled and taped? I assume there is some reduction of transmitted light.

 

I ask, because I am trying to assemble a telescope that calls for a lens thicker than any affordable glass, but less than twice as thick as what I have.

 


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#2 GlennLeDrew

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Posted 10 January 2017 - 11:12 PM

If the convex curves on the two PCX lenses are the same as on the bi-convex lens, *and* the combined thickness is the same, then indeed those two PCX lenses when placed in contact at their flat sides will be effectively the same as the singlet. As long as the separation at the interface is just about zero, the exceedingly thin air gap will not alter the final, emerging wavefront to any meaningful degree. And so it's not really necessary to cement the lenses, although this would provide the best transmission due to the very low reflectance.

 

Out of curiosity, in what capacity would this lens serve?



#3 MKV

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Posted 10 January 2017 - 11:25 PM

And so it's not really necessary to cement the lenses, although this would provide the best transmission due to the very low reflectance.

Not cementing or oiling them, antireflection coating may be in order.



#4 MCovington

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Posted 10 January 2017 - 11:28 PM

Cemented together, they *are* a biconvex lens, assuming they are the same kind of glass.

 

Tell us more about the telescope!


Edited by MCovington, 10 January 2017 - 11:28 PM.



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