In my new house, the rising winter sun projects from an upstairs window on to a big blank wall. The distance is about 48'. I think it would be cool to build a little obscura lens and project a detailed image of the sun. I would like it to be about 3' in diameter which would be much bigger than a simple pinhole.
Can anyone tell me what sort of lens I would want and where I might find one for cheap. My idea is to just take a sheet of plywood and make a primitive lensboard that would sit inside the window. They we can sit and admire our heliostat.