The preceding posts cover : “C Plath sextant lives again”; “C Plath Micrometer Sextant”; “A Damaged Rising Piece”, “SNO-T Mirror Bracket Repair”, “A Worm Turns”, “The case of the broken screw”, and “Worm with wrong thread angle?”
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.
Figure 2 : top - faulty collimation; beneath - corrected collimation.
