A lens has two major types of optical aberrations, spherical and chromatic. As you know spherical aberration is when the figure on the glass causes light not to come to the same focus position. A fully reflective system has no color error just errors caused by the figures.
In an achromat even if the lens is perfectly figured as per the design to have no spherical aberration, that only happens at one wavelength which is usually in the green. That is why you test with a green filter to isolate that wavelength. With the blue and red image, they that don't come to the same focal position and two these wavelengths will show spherical aberration even thou the lens is perfectly figured in the green. This is called spherochromatism ie spherical aberration as a function of wavelength.
So even if you have a perfectly figured achromat in the green you still have the problem that red and blue don't come to same focus. So you have a wavefront error. You have a sharp image formed from the wavelengths that come to focus well enough to form a diffraction limited image swimming inside a blurry image caused by the other wavelengths that don't come to focus. That is what causes the purple halo around bright objects.
Now think about what happens when the lens is not figured correctly ie it shows spherical aberration in the green. Now the image formed in green light has errors but this also translates to the red and blue image which already have spherical aberration. So these wavelengths have increased in spherical plus it causes more defocus across all wavelengths. You already have chromatic aberration to start and now it has also increased.
So to summarize in a lens, spherical and chromatic go hand in hand so if one changes so does the other. Hopefully this makes sense and explains my above statement.
Ok, now I understand your comment. You were speaking more broadly to include other wavelengths other than green. I thought you were referring to longitudinal chromatic aberration in just green , which didn't make sense to me since the image was taken in a very narrow band around green light.
Regarding spherochromatism. It's been my experience in DPAC that achromats, which can have boat loads of longitudinal CA, especially with a fast achromat, have comparatively little spherochromatism relative to the longitudinal errors but also when compared to some faster APO's. I just posted in the refractor forum some DPAC shots of two 6" F10 achromats. While the longitudinal CA is readily seen, especially in blue, both samples show very little spherochromatism. I suspect that the under/over correction seen in Roberts scope in green, may very well carry over into both the red and blue. But that's a guess.
Edited by Jeff B, 28 June 2020 - 10:30 AM.