Something I always found interesting was descriptions of the fast
lenses used for manufacturing the masks for semiconductor chips. I have not
read about them recently so this may be way out of date but... : These are
extremely expensive very limited production lenses (Usually Ni*on) with
diffraction limited performance wide open and max apertures of less than F1.
They are aspherical lenses and are optimised at two wavelengths. One for
actual exposure at a short wavelength and the other to allow focus to be
checked visually by the operator. No need to worry about chromatic aberation
as they use monochromatic light sources. Because the performance is
diffraction limited there is no reason to stop down as the performance would
only get worse, which is why they try for as large an aperture as possible. I
seem to remember that max apertures in the 0.5 to 0.8 range are normal.
Tim Hughes
Hi100@xxxxxxx
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