geophilip06

I know this is an old post, but this explanation may help: https://www.telescope-optics.net/polychromatic_psf.htm "Since the RMS wavefront error is proportional to the P-V error, the actual wavefront error for an achromat of given aperture changes approximately in inverse proportion to the square root of its focal ratio. Actual chromatic error in an f/10 achromat is only 0.71 of that in an f/5, but in the latter it is also only 1.4 times larger than in the former. This is what the advanced optical design software programs, using diffraction calculation, imply (note that the value of PPDI in the visual range doesn't change with scaling doublet achromat while keeping the focal-ratio-to-aperture ratio F/D constant: 100mm f/12 has identical PPDI as 200mm f/24)."