1956 C. Plath Geodetic Sextant

I've obtained this geodetic sextant in 2008, but first needed some time to clean it from corrosion. It involved complete dismantling of the sextant and removing all dirt, corrosion and loose paint, which took a mere five hours. As I decided not to repaint the instrument it still is not the prettiest thing, but - alike the Observator Geodetic sextant in my collection - it is an exceptional instrument as it has a centesimal scale (this means the scale is divided in the 400grad system, where 400grad equals a full circle).

The drum micrometer is divided in centigrad (one hundredths of a grad, see figure 5) and can be estimated another decimal although that will be beyond the instrument's accuracy. The sextant can measure angles up to 144grad.

In order to be able to measure even larger angles a pentagon prism could be fitted in front of the horizon mirror (see figure 6 and figure 7). In this way the measured angle would increase with 100grad, giving the instrument a working range of 100-244grad. Sadly enough the original prism is missing, but I made one to experience how it worked (see below).

Alike the Observator Geodetic sextant it has six legs to rest on (one of them doubles as a support for the handle), three of them on the telescope side. They are better placed than with the Observator Geodetic sextant where one of the legs is actually blocking a part of the view through the telescope (see figure 8).

This sextant was used on the Delta works (by the same survey department that used the Wild TC1 and Fennel/Minilir from my collection) to position gravel barges. The gravel barges would dump their load once moored along pontoons in the mouth of the Oosterschelde and it were these pontoons that were positioned using this sextant and so called "circle charts" (charts with circles superimposed on them that represent horizontal angles measured to known points with the sextant). It has been in use for this purpose until the arrival of the Trident positioning system (not to be confused with the weapons system with that same name), and when that arrived the sextant was used to check against.

The handle has a threaded hole, to make it possible to attach a perpendicular handle (see figure 2). When using this perpendicular handle the instrument can't be laid down as usual on its back, so three additional legs are provided on the front to rest the instrument on. Another feature of a typical surveying sextant is the total lack of shades. The frame is a standard sextant frame and the threaded hole used to fasten the shades is filled with a brass headless screw.

Of course the instrument and especially the prism had to be checked for index errors. The prism can be rotated over 100 grads by which it will either substract or add 100 grads to the observations. In the former way it is possible to check the index error by setting the index arm at 100 grad, while in the latter way it is possible to observe angles up to 244 grads. As the pentagon prism is set in the center of the telescope there is still some free space to be able to look past the prism and check for the index error at 0 grad as well (see figure 8 and figure 9). Several side-by-side moon shots showed that the additional index error of the penta prism was less than 0.01 grad (0.5 arcminute).

I decided to replace the missing pentagon prism with my own interpretation of it (see figure 14). Although they are still commercially available from Cassens & Plath, the price of the original prism attachment alone is however half the price of their top model sextant, which I thought was just a bit too much of the good thing.

The prism itself was quickly found thanks to late George Huxtable who kindly provided the address of the English manufacturer Edmund Optics. The prism I chose was a 25mm TECHSPEC® High Tolerance Penta Prism with a 1 arcminute angular tolerance. I made a fitting of 2mm brass sheet and a mount of 4mm brass angle bar (see figure 15). The fitting was hard soldered and all other components were kept in place using brass pins and fastened with either stainless steel or brass screws. So far I did not yet give it a finish, that will probably have to wait until I decided whether or not to repaint the whole instrument.