This is really exciting stuff Alex. Great job. Can you spill the beans a little more about the exact construction of the lens, setup, optimum lens to port distances you have found etc. Is this essentially a flat port with a dioptre type lens element rather than a flat glass. Got thrown a bit when you discussed the lens threads; is there a component that screws to the lens as well?
Yep. While the port looks flat, it is actually slightly convex on the outside and strongly concave on the inside. I think it is actually made up of two elements. The second part (which mounts on the lens) is convex on the outside and concave on the inside. I have measured the dioptre strength of the internal lens - and tried replacing it with a simple dioptre lens (of the same strength) and it did not work well (reduced the optical performance to that of the dome). I am not having a new internal lens made that is the same dioptre strength and curvature (inside and out) as the current internal lens. This will allow me to use a much wider range of lenses. I am currently limited by the relatively small physical diameter of the internal lens.
The thread on the outside of the internal lens is a non-standard size and I don’t have a step ring that will hold it. I will have to have one made. So at present I am holding it on with blu-tac (a putty), which works well, but can cause problems of misalignment through carelessness or vibrating boat journeys.
The other modification I need is a lens hood. Both for shading and to protect the optical surface.