NavList:

Hi Paul,

I am intrigued by your Freiberger sextant with a pentaprism. Interestingly, this surveying version of the Freiberger sextant provides a locking mechanism to exchange the pentaprism against a standard set of horizon shades. I have not seen a similar arrangement anywhere else. Cassens & Plath mount their pentaprism using a spring-loaded bayonet socket, as described on https://sextantbook.com/category/sounding-sextants/, a system that is still sold by Cassens & Plath until today and can be retrofitted to any of their sextants (though the Cassens & Plath pentaprism with mount will cost as much as their complete surveying sextant without pentaprism). It should be possible, in theory, to devise a set of horizon shades that would fit into the same bayonet socket, but I don't think Cassens & Plath has ever offered anything like this. One could, of course. unscrew the pentaprism mount from a Cassens & Plath sextant and exchange it against a set of horizon shades, but this would require significantly more effort than the Freiberger's "plug&play" system. The original pentaprism was not available for the instrument described on https://dehilster.info/geodetic_instruments/1956_c_plath_geodetic_sextant.php; therefore we don't know what mounting system C. Plath used (possibly the same as Cassens & Plath).

Another interesing aspect of your Freiberger sextant is that the horizon shades are mounted in the reverse position, and thus move in the opposite direction, as compared to the standard version. Also of interest is that your 1974 surveying sextant uses the same second-surface index mirror as all other Freiberger sextants from before the 1980s (Freiberger has switched to first-surface index mirrors sometime in the very late 1970s or early 1970s and still uses them today, while Cassens & Plath has been switching back and forth over the years and currently uses second-surface mirrors as index mirrors for all of their models).

Did your sextant come with any kind of certificate(s)? If yes, I would be really useful if you could post photographs. The type of certificate might give hints about the sextant's previous use, and it would be interesting to see whether the pentaprism's error (deviation from perfect 90 degrees) is noted. Do you have any information about the sextant's previous use as a survey instrument, or about the ship or company for which it was employed?

I'd also be curious where you discovered such an unusual instrument. Can you reveal how much you paid for it, and in what condition you found it, given its age?

Regards, Martin