Checking Index Alignment on a Davis Mark 15 Sextant

When the sextant reads 0º 0.0’ on the micrometer drum, the index mark on the on the movable sextant arm should be perfectly aligned with the 0º line on the non moving sextant arc. Usually these are well within workable specs, but once in a rare while we see them not quite right, meaning a misalignment slipped through quality control. On some metal sextants you can actually loosen a set screw on the micrometer drum and adjust this, but on the Mark 15 these are glued to the worm gear axis rod. Here we compare one that is in alignment with one that is notably off, though still quite usable. The Mark 15 and other Davis sextants are available at https://www.starpath.com/catalog/accessories/1840d15.htm