Tamaya Collimation Blunder

2 09 2011

A comment in February this year on NavList about the Tamya Regulus sextant set me wondering, as the Tamaya sextants I have examined seem to be well-constructed. The writer commented on problems people had with the Regulus pattern at a nautical training establishment in the 1970s, so when a Tamya Regulus II recently came into my hands for overhaul and restoration work, I looked at it with unusual care. It seems to have been well-constructed, following the pattern set by C. Plath many years ago. It has an aluminium alloy frame with bronze rack, large mirrors and shades to match,  an adequate 3 x 40 Galilean telescope, a very good scale illumination system and a switch that is accessible and easy to overhaul. I was as puzzled by the adverse comments about the instrument by the time I had put it together again – until I came to align the telescope.

The telescopes of many sextants can be collimated, that is to say, the axis of the telescope can be adjusted so that it lies parallel to the plane of the arc. A lot of modern sextants do not have this feature, as the effects of mis-collimation have relatively little effect on the accuracy of observations, unless the observed angle is high or the angle of misalignment is great. For example, if the observed angle is 60 degrees and the misalignment is 55 minute, the error will be only half a minute. In fact, the error is proportional to the tangent of half the angle of observation and to the square of  the angle of misalignment in minutes.

Usually, the telescope screws into a flanged ring, and two screws allow the ring to be rocked about the rising piece, with two cone-ended screws for an axis, as shown in Figure 1.

Figure 1 : Collimating rising piece exploded

After I had adjusted the mirrors, I moved on to check the collimation of the telscope and to my surprise found that it was not possible to do so, as the adjusting screws rocked the telescope up and down, parallel to the plane of the instrument, rather than at right angles to it. Figure 2 shows at the top the rising piece as I found it and beneath, the rising piece as it should have been.

This led me to look at another sextant of identical construction and undoubtedly made by Tamaya though bearing another name. It too had the same mistaken construction as did one currently for sale on e-bay and named Tama-Sokki. I promptly re-arranged matters by plugging the tapped holes for the adjusting screws, tapping holes for the adjusting screws where the seats for the cone-ended screws had been and rotating the telescope ring through 90 degrees (Figure 2, lower half). This modification requires only two grub screws and can be done with hand tools alone. One can only guess how Tamaya overlooked this blunder. Probably someone in the drawing office got a bit confused, noone in production noticed the error and, it seems, neither did a great many sellers and users of the instruments. My impression is that many users never attempt to check for anything other than index error. There is something to be said for noting small index errors rather than constantly fiddling with the adjusting screws, but personally I would always check the adjustment of any instrument that I was going to rely upon, correct large side and index errors and at least glance at the telescope to see that it had no obvious lean to or away from the frame. Any good book on the sextant (like The Nautical Sextant) and many manuals of navigation will tell you how then to adjust the telescope collimation.




2 responses

3 05 2014
Mike DeBurgh

When I acquired my Tamaya Model No. 633 Model 2 (1975) I assumed that what you call “rocking screws” were lock screws and that your “pivot screws” were the adjustment screws, meant for adjustment perpendicular to the frame.

3 05 2014

The pivot screws are the cone-ended screws about which the telescope ring pivots and the rocking screws are the screws that cause the ring to pivot and which also lock the ring in its final position (see Figure 2).

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