[bojan]

I know I am spending too much time with this lens (one of my friends on other forum suspects I am obsessed with cats..).

Since I have two specimens now (Opteka DG and recently acquired Roninon DX brands), I started to swap elements to determine which one is contributing to Opteka DG coma.

First, I tried to vary the distance between two elements of Opteka FF assembly (drawing is in the first post of this thread), but the results were inconclusive, so the next step was to swap only individual elements of the FF group.

The easiest was to swap the 2nd element of the FF group (negative doublet, accessible from the back without dismantling the rest of the lens), and the improvement was much more dramatic - almost acceptable, shown on attached image (visible CA may be atmospheric? Arcturus (imaged) is quite low above horizon..).

The next step I am planning is quite unorthodox - I will try to replace the whole FF group with Meade 2x Barlow, it will fit nicely in place with the help of adapter (to be machined from Delrin ø40mm rod). I am not expecting a lot but since the optical designs are similar, barlow should work somehow (provided I can reach focus with Canon camera).