Re: ATM Ready for a refractor.

[Anthony Stillman <atmer@flash.net>]

>...equalize the focal lengths for C (red) and F (blue)

This, for a doublet, is termed achromatic(1)

When a doublet is undercorrected the wavelength of minimum focus moves
toward the blue.  Correspondingly when overcorrected, toward the red.
Moreover, when overcorrected a somewhat smaller portion of visual
wavelengths can be said to have less residual secondary spectrum.
Newport's design may be optimized to work with a minus violet.


Anthony


1) Kingslake '78