[LewisM]

Finally acquired a Pentax SDUF II f/4 100mm astrograph (the fore-runner to the modern Vixen VSD 100 f/3.8 system, which is 5 elements instead of the Pentax's 4). Should keep me entertained for a while...hopefully.

Being a 4 element Petzval, with an SD element (Super Extra-low Dispersion glass), it is well corrected throughout the spectrum - especially red - EXCEPT for blue. Blue in images acquired with these show blue flare/bloat/halo.

Will a Baader semi-apo filter help in IMAGING (I don't care visually) to reduce the blue menace?

I ordered one from myastroshop anyway -guess it can't hurt to try!