20170923 M45 Pleiades (200om)

Interesting shot..I'm curious about the colours and surprised to see so much in the way of purple haloes for a proper camera lens. Is the didymum filter fooling the focus?

RL

Not a focus problem, it was dead on as far as I can tell. Such saturated / somewhat wrong colors come from applying a star-based B-V color calibration on an image with stars bloated by halos of aberrant colors.

Colors and halos are exactly the same without didymium filter, but the filter allows me longer subs without saturating the background.

IMO my lens model is of the poorly-coated variant (Oly made another variant of same 200/4 but with MC). Hence CA produces strong halos of different size for blue (bigger) and red (smaller). The end result depends on original star colors, but typically shows outer blue then inner red or purple.

I hope to try a technique based on star filters to remove the halos someday...