Back to Zeta Sagittarii. Looked at it tonight for the first time in quite a while. Used the 140mm refractor. I had another look at the ephemeris and I think I have a typo - or maybe a slip of the calculator - in my note above. After careful assessment of the ephemeris (from the Grade 1 orbit) current separation is 0.45", not 0.42".
So I thought I'd use it for a break-the-0.5Rayleigh-barrier attempt. Seeing was better than various recent nights. I was pleasantly surprised to find a very slight elongation at 400x, and more obvious (though still slight) at 570x. Other nearby stars (single) did not show anything but neat'n'round. So, that's about 0.45-Rayleigh, my best yet for the particular scope. Previous best with it was with Zeta Bootis a while back when it was at about 0.48", likewise an equal pair, bright, and using 400x/570x.
If I get opportunity (another steady night) I will try it with the Mewlon 210. Rayleigh on that is 0.66", Dawes is 0.55". So it won't be a clean split, but a more obvious elongation. At 0.45" it will take a 10-inch for a Dawes tangent discs "split", or a 12-inch for a Rayleigh criterion split. That's now. By next year it will be closer still. By September 2020 about 0.33". It changes fairly quickly once it gets well into the closing swing. Minimum separation, below 0.20", appears to be in 2022-23. After that it widens again.
By the way, Zeta Sagittarii is one of the 175 doubles featured in the new Argyle book, An Anthology of Visual Double Stars (#144). Orbit diagram and ephemeris given there.