I'm coping with unpleasant star halos and possibly focus issues with my Optolong blue filter.  My theory is that my ES 127ED apo refractor is not well corrected below 400 nm.  The blue filter's cutoff is around 375-380 nm, so I'm thinking ultraviolet is my problem.  The easiest way to mount the UV/IR filter is directly on the ASI 183MM - Pro camera, which is threaded to accept 1.25" filters.
My question, though: does it matter whether the UV/IR filter is in front of or behind the blue (or any other RGB) filter?