A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Bill Morris
Date: 2018 Apr 7, 14:50 -0700
Frank and Ken's comments about accuracy are legitimate. Any metal-framed sextant in good adjustment will give adequate accuracy at sea, though if you are planning to do lunar distances, it pays to get one of assured quality. Ken's remarks about most certificates being fake may well apply to some of the Tamaya clones (which are themselves clones of C Plath instruments) and possibly also the Astras. However, C Plath certificates can be relied on. When they say "...free from error for practical use" they mean that no error exceeds 24 seconds, while the manufacturing standard of the Soviet era SNO-T is the same, and possibly better for earlier instruments. I have calibrated three SNO-Ts and not found them wanting. Freiberger certificates have not appeared to me to be fictional either, as I have seen some admitting to errors in exess of 60 seconds. I assume they would not admit to such errors if writing fiction.
Bronze frames can be bent by dropping, whereas aluminium alloy frames are more rigid, as well as being lighter. There does not seem to be much to be gained by the hybrid instruments which have alloy frames with bronze racks. I suppose there was a perceived risk that a hard brass or nickel silver worm might not wear well running against an alloy rack, but most of the last generation of sextant designs had the worm running directly in a rack machined into the alloy frame.
There seems to be no risk of prismacity of mirrors and shades in sextants made in the last century, but telescopes have improved greatly since about 1935, beginning with German and Japanese "star" or Galilean 'scopes with their greater objective diameters (and mirrors to match), which translated into greater field of view for a given magnification as well as better contrast at the horizon. The Soviet SNO-T followed suit with its superb optics. Ken's comment about all 'scopes being made in China should surely have included the word "now".
Cassens and Plath has not been the same company as C Plath for many years and I am not at all sure where their instruments and telescopes are made.
In your place, if on a limited budget, I would look out for either a second hand C Plath sextant in good condition and made in the last 50 years, though I would avoid their "Navistar Professional"; or a SNO-T, making sure that it had both telescopes, or at least the (conical) Galilean and a full kit of adjusting tools except for the screw drivers. If buying off the internet, do ask the buyer to include packaging within the instrument case so that it does not damage itself if it should break loose from its moorings during "handling".
May I without false modesty refer you to my web site www.sextantbook.com where you may find more than you wish to know about various sextants and their intimate anatomy. I am happy to be contacted by email if you think I may be able to help you more with your choice.