NavList:

Hi Paul,

Thank you for posting the picture of this interesting Russian (Soviet?) sextant. According to the packing list of the box, this model is designated SP (СП in Cyrillic). In an earlier post to this list (http://fer3.com/arc/m2.aspx/SNOM-CHOM-sextant-booklet-manual-completed-IvanL-nov-2014-g29400), I found the manual for the SNO-M, which also covers to the SP sextant. The manual is now also available as a pdf from https://hydrography.pro/presta17/index.php?controller=attachment&id_attachment=53 (since you refer to its certificate, it seems that the manual is already available for your SP sextant).

According to the manual, SP stands for Sekstan Promernyj (Секстан Промерный) "sounding sextant," as opposed to SNO, short for Sekstan Navigacionnyj s Osvetitalem (Секстан Навигационный с Осветиталем) "navigational sextant with illumination." Also according to the manual, 2 arc minutes would be quite a bit better than the specified maximum eccentricity of +/- 3 arc minutes (+/-3.4 arc minutes above 120 degrees). While there is no provision to attach a pentaprism, it must be noted that the SNO-M and SP are quintants that measure up to 140 degrees, thus already provide a somewhat larger measuring range than a standard sextant.

Though the manual is silent on this, I suspect that the two lightly-tinted shade are less for blocking glare from reflecting surfaces but mainly for adjusting the amount of reflection in the unsilvered, clear-glass half of the horizon mirror, which might be particularly useful when aligning two marks on land. Can you confirm this from your experience in practical use of this sextant?

Do you have any information about the date when this sextant was produced? I would be very much interested if you could post a picture of the certificate with the correction values.

I find surveying sextants quite an intriguing topic. As most on this list will probably know, Hubert-Jan Lekkerkerk has quite a bit of information on his HJs Boeken en Instrumenten website (https://hydrography.pro/presta17/instrumentblog/survey-sextants-in-the-netherlands.html, https://hydrography.pro/presta17/instrumentblog/the-survey-sextant-outside-the-netherlands.html, https://hydrography.pro/presta17/25-sextants?q=Type+of+use-Survey). Pictures and information (and price) of the currently available Cassens & Plath surveying sextant and pentaprism can be found at https://shop.cassens-plath.de/en/sextants/sextants/51/surveying-sextant?c=27 and https://shop.cassens-plath.de/en/sextants/accessories/52/pentagonal-prism-for-surveying-sextant?c=60, respectively. The swappable shades/prism arrangement of the Freiberger and the single shades (rather than no shades) of the Russian SP seem quite unique among surveying sextants.

Regards, Martin