My results (TEC140, earlier model) are that focusing (better) doesn't make much difference. It depends on the lens figure. Visual blue can be razor sharp but all higher frequencies will be out of focus. Things are improved if the blue filter cuts closer to the best focus of the lens. For example my Baader filters cut around 380nm, but the TEC140 is figured to be sharp at around 430nm. I saw some slight improvement testing an Astrodon blue that cuts at 400nm, a bit better cutting at 410 with a Hutech IDAS, and quite good with some special filters cutting at 420 and 430nm.
However, after quite a bit of experimenting I have now abandoned all UV block filters. The increased exposure and difficulty of bringing out a good blue in final images seemed to outweigh any advantage I thought I was seeing. In fact the disadvantage of bloated blue seems to be very target dependent and often was not seen in the final image even though the blue subs didn't look too swift.
I'm just about to start processing my first image taken with the new Astronomik filters that claim to be designed exactly for refractors with this blue halo issue. The higher cut blue is clear from the attached photo. It looks to be about 420nm. Interestingly I have not noticed that I need longer subframe integration in blue as I did using 2" UV block filters in front of my Baader blue filter.
I'm most curious to see how these work in my system!
http://www.astronomik.com/en/photogr...r-filters.html
Peter