The long term plan was to always use the Ca K filter as a module that could be swapped around scopes, and I knew that in the long run I'd be wanting to use it with scopes of greater aperture than 40mm, more likely around 100mm. So I reused an existing aluminium adapter (the Ha & Ca K PSTs have the same thread) Part 'C' in the photo is the CaK filter, then there's an aly adapter and part 'E' is the original blocking filter from the Ca K PST. Unfortunately this BF only has a diameter of 5mm which is as large as it needs to be for the original 400mm FL PST, so it works, but vignettes the image at focal lengths longer than 545mm.