Hello,
I have been testing a Tamaya MS 833 which I recently borrowed. I corrected perpendicularity and side error as indicated in the manual. The residual index error is +1.1’, computed by superposing upper and lower limbs of the Sun and Moon, which give the same result.
The instrument appears to be in very good conditions, and the reported instrumental error is 0.0’ for all reported angles. 
I have been testing it by measuring the angles between pairs of stars and comparing the measurement with the prediction from Stellarium software, which incorporates refraction effects. The sextant appears to systematically overestimate the Stellarium value by ~ 1’. Do you have an idea of why?
Here is my guess: when I superpose the direct image of one star and the double-reflected image of the other one, their angular separation depends on the orientation of the sextant, see the figure attached.
I first orient the sextant in such a way that the two stars are far from the line separating the non-coated from the coated half of the horizon mirror (A), which means that they are close to the right edge of the coated part. If I rotate the sextant in such a way that the stars move to the middle of the mirror, the separation increases (B). I think that the solution would be to make the measurement by keeping both stars close to the middle of the horizon mirror? 
Try doing a few noon fixes. Easy enough to do quickly. Then see how off you are.
I traced three lines of position with a Sun, Moon and Arcturus sight, by using a liquid in a pan as artificial horizon (the horizon is not visible in my place). I am ~ 3 miles off.
Did all 3 intersect or was there a cocked hat? Either way 3nm is acceptable for a landfall.
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I was going to suggest checking the calculated declination of the sun at noon against what the sextant reads. If you get the same error a few days in a row, then that is what the correct Index Error.
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There was a hat, but I am not satisfied with this criterion, it is too indirect.I would like to pin down the instrumental error directly, with a measurement simpler than that of tracing a LOP, and which involves less potential accumulated errors.
Thank you. How is this different from checking the angle between two stars and check whether I get the same error for multiple pairs of stars?
I suppose the other would work as well. Noon sights just seems like an easier process to me. Since its one of the simplest and most common sights, its one that a mariner using CelNav would be most familiar with. Noon Fixes and Sun Fixes in general are far more common to use than a star fix. And the moon moves too quickly to be very accurate unless you’re very skilled. Proficiency with sun fixes is the first step. Also you get the benefit of daylight for clear horizons.
