Quote:
Originally Posted by Ukastronomer
Well, YES or NO can a doublet be an APO scope ?
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Well it really depends on how exactly or inexactly one cares to define an apochromat:
Is it defined by
1. the number of lenses in the design or
2. by how well it corrects for aberration (typically spherical and chromatic) ?
Some would say that
1. if it has 3 zero crossings on the focus v wavelength curve (3rd order cubic function) then it's a triplet and apochromat and
2. if it has 2 zero crossings on the focus v wavelength curve (2nd order parabolic function) then it's a doublet and achromat
3. if it has 4 zero crossings on the focus v wavelength curve (4th order quartic function) then it's a superachromat
Others simply say that the apochromat corrects better than the achromat and leave that as some sort of definition.
To me the optical performance matters more than the specific design or moniker conferred on a design. "Back in the day"
the
achromat was taken to mean a optic without colour errors/fringes/abberation and it certainly would have appeared so in comparison with more commonly used singlet lenses of similar focal length/aperture at the time. Enter a new design that has better correction still for colour and spherical abberation and it's differentiated by calling it a
apochromat, typically these were triplets.
Just because something is called an apochromat doesn't per say make it better than an achromat, even though it may be that a particular design usually confers a notional performance advantage. Only objective optical performance matters
So to answer your question: YES, NO and MAYBE
Best
JA