Pentax infamous barrel notch
The problem with pentax scopes is that the eyepiece socket ,has a notch in the bottom that prevents many eyepieces to sit all the way in,preventing them to reach infinity focus..Yes ,I need infinity focus to bird,of course I do. Specially with a scope!!!..I had to cut a niche in my old Swarovski Astronomy Zoom barrel,many years ago,to reach infinity in my Pentax PF65ED.It was a fixed 1 1/4 barrel and the field stop was way above the end of the barrel,so no problem.With the baader I don't know,but I think would be the same.I don't think the field stop would create any problem,as the Kowa tsn3 narrow neck did ,to prevent insertion.If the astro barrel prevents full insertion ,the problem would be easily and safely solved cutting a small niche in the bottom..