as others have said, it will almost certainly be CA - APO doesn't mean
no CA, it just means
less CA (in general). Worth a look at the spot diagrams in fig 149 in
http://www.telescope-optics.net/semi...o_examples.htm - even the most highly regarded APO designs (eg Petzval, fluorite triplet) spray a little bit of energy out a fair way from the best focus, resulting in halos. A minus violet optical filter could possibly help remove the worst of it without messing up the colour much. I think the halo reducing filter in StarTools is called fringe killer - have never used it, but
think that it attempts to remove and replace data that falls within a narrow spectral band.
Of course, you could just ignore it and accept that this is what your system does - doesn't mess up the image a whole lot.
regards Ray
