NavList:

Marty, you wrote:
"The sextant is a brand new Ogawa Seiki made in 1984, looks very much like the Tamayas."

Aha! That's a Tamaya-like. There are about a dozen different brands that used Tamaya parts in this period. They are very good sextants, and they're often available in near-new condition at excellent prices. But I have found that many of them have fairly significant arc error. This is the error that used to be tabulated in calibration certificates. It varies along the arc. But fear not. You can correct for this by building your own calibration table. One way to do this is to shoot lunars. Another approach is to use a perfectly level artificial horizon. You should discover that you get consistent errors, like the ones you posted the other day, for specific altitude ranges. It's not likely that these will exceed 3 minutes of arc, but that's more than we should have to put up with in a modern metal sextant. And besides, it's a fun and interesting project! Make your own calibration table. After that, your sextant is as good as the best in the world.

Frank Reed