NavList:

Hello Richard. You wrote:
"As far as frame-parallel, if you get the complete SNO-M set, there are two little 'dioptres", level-sight pieces, to set on both ends of the arc scale with the sextant on its side. This is the 1967 Soviet version of your laser level. These little pieces also help with index mirror verticality. Did other makers provide such a self-sufficient gizmo? "

I believe that there is good evidence that these little sight pieces have been co-opted from their original purpose. I'm attaching a portion of a plate from a book by George Adams on sextants published in 1789. Those sight pieces marked "Fig. 2" and "Fig. 3" probably look a lot like the ones accompanying your Russian sextant, right? To get the index mirror perpendicular he advises placing the sight pieces on the arc and then lining up a direct image of one with the reflected image of the other. That's standard; it's the original purpose, and you'll find pretty much the same advice in every book on sextants published over the past two centuries, but few modern sextants come with those special pieces. You can buy two little brass cylinders from Celestaire just for this purpose, delivered like fine jewelry in two little velour bags --a gift for the navigator who has everything. When Adams described how to collimate the telescope back then, he didn't mention those sight pieces or any tabletop arrangement and instead described the rather difficult trick with an angle between the Sun and the Moon that I mentioned previously. That's the only recommended procedure for telescope collimation that I have seen through the 19th century. I have not seen the tabletop trick written up in any pre-20th century navigation manuals.

By the way, do you have a copy of the SNO manual? There's one in the NavList archives somewhere.

-FER

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