Re: [ATM] Lens Wedge

[James Gort <jgort@oxford-observatory.org>]

Anthony,

That's what I was looking for!  One thing that the books say is that 
wedge is difficult to control (you verified that).  I would add that 
it's equally difficult to measure - I have an elaborate turntable and 
multiple dial indicators, and take several readings in various 
positions.  The standard deviation is about .0001.   For the master 
makers (the Clarks, Brashears, etc.) it must have been more difficult, 
since the measurement technology was more crude.  Of course, localized 
polishing of the doublet (or shimming) could have been used by the 
masters to minimize the effects of residual wedge, but I think your OSLO 
demo proves that it probably wasn't necessary.  I often wondered how the 
"0.001 inch" value came about.  Some one might have proposed it and it 
became accepted. 

I don't have the updated version of Texereau, but Jan Bentz tells me Tex 
might have calculated allowable wedge for a plano parallel window.  I'd 
be interested to see that if he did.


James

Anthony Stillman wrote:
> James,
>    
>   I too looked through a bunch of books and only found cautions and stringent criteria for achromatic lens wedge.  No qualitative equations.
>    
>   So I plugged a few frounhofer designs into OLSO and started tilting surfaces.  Qualitatively I've learned that a little wedge is no big deal.  This is particularly so of the crown element.  The effect was about five times worse for the flint element.
>    
>   In both the 150mm f/15 and the 125mm f/18 design (BK7, F4) , one third of a degree of wedge seems to be the pardonable limit for the crown element.  This amounts to about 3/4 of a millimeter.  One half a degree is clearly too much.  The MTF shows that, as one would expect, the mid and high spatial frequencies are what suffer.  Astigmatism is the principle cause, not chromatic abberation.  Of course chromatic abberation in a doublet is pretty awful anyway.  A slightly prismatic lens doesn't seem to make it much worse.
>    
>   Lastly, in the last few years I've made a half dozen lens (four of them singlets) and holding wedge to a few mill was easy.  Getting it under 0.001 inches and holding CT was tricky but it was easier than figuring.
>    
>    
>   Anthony
>   
>   
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