Re: Updated Wiki Page: Diverger lens Residual Spherical aberration #wiki-notice


George Roberts (Boston)
 

Lol.  Basically in a Bath, there if you use a biconvex lens there is a tiny contribution of that lens to the spherical aberration term.  Typically it may add less than 1/1000 of a wave of error to the DFTFringe result.  No big deal.  It depends on things like how big and fast your mirror is and what type of glass is in the thin lens and so on.  Bruce has an equation that lets you calculate the error.  I might make a web page calculator where you enter the terms but the error is so small it's probably not worth the trouble except...

If you use a PCX lens instead of a BCX lens (PCX - plano convex lens - that means one side of the diverger lens is flat) then this spherical aberration grows.  It's still typically less than 1/50th of a wave but it could be significant if you have an unusual mirror.  I have a calculator here to see how much PCX will affect your mirror here - just plug in your mirror parameters:
gr5.org/bath/

However Bruce improved the equation to include the index of refraction of your PCX lens which most people probably won't know.  So maybe best to use the older version which just assumes 1.5 which is hopefully close enough.

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