NavList:

Thank you all for suggestions. As I mentioned in my first post, "we are trying to determine just how bad this design really is. You may count that as an a priori conclusion: it is bad." And that remains true. An interesting open question is whether a "bad" pseudo-sextant can fill any role. For example, could it be used for coastal navigation where an angular accuracy of +/- 0.5° might be useful? I doubt it could even reach that minimum level of usability, but we will see...

This Fake Tamaya weights about the same as a professional sextant of similar size. I'm not sure what the frame is made of. It's not aluminum. It could be steel.

The handle on this sextant has fair ergonomics. It's hard plastic with a faux wood grain. Conveniently, there is a large metal plate screwed to the back of the handle. On the plate, in engraved lettering about 1cm tall, we find the label "SEXTANT". This is convenient in case you ever suffer from "navigation amnesia". You will be able to turn this instrument over and say "Ah yes, this is a sextant. I thought it looked familiar!". Apart from this (imaginary) convenience, the giant label is a warning sign. It's like discovering a camera in a pawn shop with a big label on the back that says "CAMERA". It might be more honest if the label read "FAKE-TAMAYA".

The shades on this sextant have nice frames with good motion, but the shade material is clearly inadequate for Sun sights. With all three index mirror shades combined, the view is about as dark as a dense pair of sunglasses. Looking through the shades, I can see the large-screen video monitor across the room clearly enough to read text. It is a bight clear morning here in southern New England, and with all shades combined I can look at the backyard and see the last leaves of the season on the trees. They are darkened, yes, but individual leaves are visible. In short, the shades might be appropriate for Moon sights, but otherwise useless. Note that many sextant users have discovered that they can replace original shades with some dark material, like the filters in "eclipse glasses". And if this were the only problem with the "Fake Tamaya", it might be an option to consider.

The sextant has some sort of scope, magnifying about 2x. Although it appears to focus, and the focus ring can be turned, this does not change the optical focus at all. The lens at the eyepiece end of the scope moves in and out when the focus ring is turned, but it appears to be optically flat. The objective lens is convex and appears to be fair quality glass. I am guessing that there is a concave lens in between, presumably fixed in place. So that is bad. The scope's focusing capability is n illusion. More importantly the scope is permanently mounted on a riser that is too short. The scope looks directly at the mirrored side of the horizon glass with only a small portion of the direct side (horizon side) visible.

The sextant has an index error of over 20° (yes, that's twenty degrees). While it is possible to work with a sextant that has a very large index error like this, it is clearly a design flaw. And it's obvious. If the sextant is set to zero, then, by design, the index and horizon mirrors should be parallel. But when I place this sextant on a table after setting it to zero, it is quite clear that there is angle between the two mirrors even without looking through the instrument. There is a set screw on the back of the instrument behind the horizon glass which could be a great thing: a means of adjusting the alignment of the horizon glass by a large angle. Unfortunately, as it arrived, the adjustment is already at the limit. I can increase the index error by another fifteen degrees, but it is not possible to decrease it.

Regarding the scale on the arc, I have not yet been able to try anything resembling test sights. The instrument will need "adjustment". I did measure the angle on the scale right away, and it is at least close to 60° over the full scale range from 0°-120°.

There is a property of the micrometer sextant that is, I think, not well-known. The transition from vernier sextants to micrometer sextants required a leap of faith a century ago that many navigators could not accept, and with good reason.

On a vernier sextant the accuracy of the instrument depends critically on the engraving of the main scale and the vernier scale. The process of engraving scales had reached a high level of perfection by the early 19th century, and sextants could be produced with exceptionally high angular accuracy (in modern terms, accurate to a tenth of a minute of arc).

Micrometer sextants transfer the accuracy from the scales to the engraved rack and worm gear. All those teeth hiding on the underside of the arc, and the worm gear of the micrometer that fits into those teeth, are essential to the instrument's angle-measuring capability, and its accuracy depends on them.

I tried one simple test on this "Fake Tamaya": turning the micrometer drum by five full degrees at a time (five rotations back to 0') should advance the location of the main angle pointer on the arc by five degrees. That's simple, right? But by 25°, the main arc lined up with the primary angle indicating pointer when the micrometer read 30'. By 75°, the micrometer had been turned through 78° (78 full rotations of the micrometer drum). It's an average of 1 in 25, 1 in 20 for significant portions of the arc, or about 4-5%. The cause of this? There's no way to be sure without more testing, but it's "bad" behavior. 

—FER
(signed in my traditional style for a little inside joke... do "you" see it?)