Thanks Rick, Marco and Greg.
Rick, you are right about the central obstruction: It is 61%, making the effective f-ratio close to f/3.
Marco, rather than showing you a boring black tube, here's an image with all the structural elements before anodising. As you can see it is all made of aluminium, which is not good for keeping focus, but I had no choice due to the complexity of the design.
I had several goes at uploading a larger version of the Witch Head into my ASV gallery. There seems to be a limit on the vertical size of the images so I ended up rotating it 90 degrees but even so it is not much better than the one here.
http://asv.org.au/gallery.php?dir=Stefan Buda
Stefan