John, I currently have a similar problem of helping to delineate a large dial, this time a vertical decliner with a 17 foot gnomon which is to be carved directly into the wall of a tower. Needless to say, the wall is not absolutely flat.
My current solution has been to build a laser trigon which will be mounted directly on the real gnomon. The protractor of the trigon is 300 mm in diameter. It has a ring of holes at 1.25 degree intervals, which corresponds to 5 minute intervals on the dial face. The laser assembly locates with a pin into this ring. The pin passes though a sliding vernier, which has 5 holes set at 0.25 degree intervals, thus allowing the individual minutes to be displayed on the dial. The precision required was achieved by drilling the holes in the protractor using an accurate rotary table clamped to the pillar drill. The alignment of the gnomon to be truely polar pointing is more difficult than drawing the hourlines - as the wall isn't guaranteed flat, there is no point of reference and its presence prevent you looking up the gnomon at the pole. My solution here is to mount a laser looking down (and parallel to) the gnomon. The laser beam hits a mirror stategically placed just below the gnomon. The mirror (first surface, optically flat) is aligned to be parallel to the equator by means of a theodolite. This theodolite is co-mounted with the laser and looks at the sun though a suitable filter. Its co-ordinates are set to values of the sun's instantaneous Declination and Local Hour Angle, obtained from a portable computer. When the gnomon is properly aligned, the laser beam reflects off the mirror and retraces its path to the laser, which is fitted with a surrounding screen to show the laser spot when it is slightly in misalignment. That's the theory - the proof will come next month! Any other suggestions welcomed. John Davis
