The attachments may interest you. I used FocusMax V curve tool to make some 'CFZ runs'...where I look at the HFD plot right around best focus. It's a chance to compare theory to reality for the size of your CFZ.
I found that for a doublet refractor (with a fluorite element, and two element field flattener))...the CFZ curves for B and I are asymmetric...probably because of spherochromatic aberration.
An SCT/focal reducer...shows roughly identical CFZ curves in B V R and I.
Most of my rigs here show classic secondary spectrum when you plot the focus offsets versus bandpass central wavelength...except for one scope...a Meade...of course!
I recommend you do some plate solves of your images through various filters (R G B, luminance, and no filter)...and I bet you'll find that the image scale (focal length) differs by a small amount.
We'll see if my hunch is right....