I already had a Baader UV & IR rejection filter. Unfortunately as you can see from the diagram it cuts off the light at around 405nm, so that's not going to be very good for passing light at the Ca K wavelength of 393nm. As usual Mr Baader was spot on. I checked it and there was no light passed at 393nm. So I searched for another UV rejection filter that wasn't quite so aggressive as the Baader filter.