Re: [ap-ug] achromat question


Mar 2, 2004

 


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#21423 Mar 2, 2004

Theoretical question...

If you have an achromat (non apo) that can bring one or two colors to

a very sharp focus does this mean automatically that the other

visible colors will also come to fine focus (although at different

focal lengths)when isolated.. or can an achromat only produce a good

image for one or two colors. In other words, can you use various

filters to bring every color to a sharp focus individually even

though they don't come to the same focus point or do other lens

abberations affect different wavelengths individually.

Dale



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#21424 Mar 2, 2004

Theoretical question...

> If you have an achromat (non apo) that can bring one or two colors to

> a very sharp focus does this mean automatically that the other

> visible colors will also come to fine focus (although at different

> focal lengths)when isolated.. or can an achromat only produce a good

> image for one or two colors. In other words, can you use various

> filters to bring every color to a sharp focus individually even

> though they don't come to the same focus point or do other lens

> abberations affect different wavelengths individually.

> Dale





No.



Spherochromatism defeats you. That is, spherical abberation which

varies by wavelength of light.



Mike



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#21427 Mar 2, 2004

In a message dated 3/2/2004 3:00:47 PM Central Standard Time,

direland@... writes:



> If you have an achromat (non apo) that can bring one or two colors to

> a very sharp focus does this mean automatically that the other

> visible colors will also come to fine focus (although at different

> focal lengths)when isolated.. or can an achromat only produce a good

> image for one or two colors. In other words, can you use various

> filters to bring every color to a sharp focus individually even

> though they don't come to the same focus point or do other lens

> abberations affect different wavelengths individually.

>



In long focus achromats, every color can be brought to sharp focus, with a

qualification. The further from the central wavelength that you go, the narrower

is the range that will be in focus. This is especially true in the blue end

of the spectrum. If you have a blue filter with a bandwidth from 4300nm to

4800nm, then you may be able to focus the 4800 portion sharply, but the 4300 will

be grossly out of focus. If the achromat is short focus, further deterioration

in the blue and red will occur due to spherical aberration

(sphero-chromatism).



Roland Christen





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