NavList:

Tinkering around with mirror and filter diagrams makes me feel that the beams would only move in unison as the sun moved across them (the thing that makes a marine sextant fore & aft tilt proof).  However, the wavelength differences in the two colours might be a clue.  If I had time, I’d look at colour splitting and recombining with prisms about which I know sweet FA.  However, this is only one third of the problem.  The Sun moves at 15 degrees of arc/hr, or 15’/minute, or 15”of arc/second of time (You can actually watch it moving though a periscopic sextant), so for one second of time accuracy you also need to set up the device facing due south to ¼ minute of arc and position the units on the board with the order of ¼ minute of arc accuracy (although tweaking the first might take care of the second).  Have you worked out how to do this Geoffrey? 

And do you know, since my latest MS ‘Office’ update someone called Al has appeared inside my PC who reads what I type and suggests in a side panel where I might find solutions.  He also disabled my page zoom until I worked out how to enable it again. What is the world coming to?  DaveP