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Colin,
For a good achromatic the spot diagrams for red/green/ blue will all show tight focused results - when the chromatic aberration curve is taken into account.
I can cleanly focus target stars from CaK (393nm) through to Ha (656nm)with the achromat in the spectroscope. Yes, there has to be re-focusing between the wavelengths, but when this is done the image and result is very sharp.
(Spherical aberrations could be an issue......)
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