I have been following Thomas's construction thread for the "Unseen University" with great interest, as well as some other threads on different forums. In photos of the Observatory, I have noticed a dark helical stripe up the side of the tower, which has seemed a little odd (but then on Discworld everything is a bit odd!) Now I have come to it in my own construction, I see that the stripe is the printed representation of an external staircase. This seems to me something of a shame, as in other parts of the model the 3D shape is well represented.
I have decided to try to model the staircase fully, so here is the story of my attempt. The strategy will be to have an inner cylinder to form the inner wall of the stairs, fix a helical staircase and "ceiling" to it, then wrap round an outer skin. This outer skin will be coloured and textured using a scan of the original skin, so needs to match its shape. The inner skin is essentially the same but reduced in width. For the ceiling I should like to have a smooth ruled surface between the walls, but this is not developable to a flat sheet, so I shall use more steps, with a narrow smooth edging which can probably be stretched into a helical shape.
Alan