This could be the edge of an alidade or a similar device. The "alidade" is attached to the back face. Along one edge there were two sights forming an alidade. The sole purpose of the spokes was to support the pivot point for the alidade. Alidade D has vanes without any openings. With skill, this sort of alidade can yield very precise measures. On the same middle the alidade with two other sights is fitted. In smaller instruments, an alidade could be used. Observations were typically made with an alidade, though newer versions could use a telescope. The alidade was then aligned with the second object as well. This type of alidade could be found on a plane table, graphometer or similar instrument. Alidade E is a representation of a very interesting design by Johannes Hevelius. The alidade was originally a part of many types of scientific and astronomical instrument. An alidade can be seen in the lower right illustration of the Persian astrolabe above. The alidade was free to rotate about a pin through the centre of the instrument.