1) How the Sextant Works
This sextant works in exactly the same way as all the older sextants. Light from the sun bounces off the index mirror, to the horizon mirror and then to the eye. The index arm is rotated until the bottom edge of the apparent image of the sun reflected by the horizon mirror is lined up with the horizon. The angle is then read off the sextant dial. (For a more full explantion of how a sextant works see http://www.tecepe.com.br/nav/sextantflash.html).
First off:-
What is a Micrometer Vernier Sextant, and how does it differ from a normal sextant?
Think of a sextant with a normal vernier to read the minutes of arc at the bottom of the index arm. Now cut gear teeth along the bottom of the arc of the sextant and add a threaded wheel like a worm wheel perpendicular to the teeth. This is the 'micrometer' section of the name. If the teeth are accurately cut, and the worm wheel is accurately threaded, you will be able to finely adjust the micrometer wheel and directly read off the minutes of arc from engravings on the micrometer handle. If a vernier is then added close to the micrometer wheel, it's comparatively easy to make a micrometer vernier.
How does the micrometer work?
Inside the index arm is a small mechanism, as shown below (with the index arm plate itself hidden):-
The Micrometer wheel and toothed wheel are firmly connected to the micrometer shaft. The micrometer shaft passes freely through the middle of the vernier, such that for each turn of the micrometer the toothed wheel advances one tooth on the gear teeth. The spacing of the gear teeth is one every 1.5mm, and the radius is such that this corresponds to 0.5 degrees of arc. So, for each turn of the micrometer the index arm moves through 0.5 degrees – and due to the way sextants work this corresponds to a sight angle of 1 degree.
How does the quick release work?
When the quick release mechanism is pushed in the direction of the arrow, it pivots about the quick release pivot point.
This causes the micrometer shaft to rotate about the pivot, and disengage the toothed wheel, as below:-
This allows the index arm to be moved rapidly to roughly the correct angle for coarse adjustment. For fine adjustment, the release lever is returned into engagement by a spring (not drawn). The release lever goes through an arc of 20 degrees, while the micrometer shaft rotates through only 5 degrees.
Temperature Compensation
I've seen a comment that temperature compensation is not catered for in this sextant. Well, yes it is and no it isn't. I'm sorry that doesn't make much sense, but I'll explain. It is designed from scratch to be temperature tolerant, and (in effect) self compensating to a great degree.
The sextant is all brass (as designed) except for one or two small pieces. This means that the thermal expansion rate is the same for all parts - something that is actually critical for maintenance of accuracy. I will consider the individual parts crucial to the accuracy of the sextant below:-
The Frame
The frame of the sextant will expand as if it is one piece in all directions equally. So, if you consider the pivot point as being fixed, the scale and gear teeth will become further away from the pivot, and wider apart. If you calculate the ratios, you will find that the angle between the graduations and gear teeth from the pivot remain constant.
The Horizon Mirror
The horizon mirror is attached to the horizon mirror support plate by brass screws and on to the brass frame. Once again, the thermal expansion of the unit will keep the mirror at the same angle from the pivot regardless of temperature, so no errors are introduced. The only thermal error introduced is the use of the 16swg stainless steel mirror. The difference in expansion expansion over 0.064" between brass and SS will cause an error of 0.000058mm for every 10C (less if you use thinner SS for the mirror). This calculation takes a co-efficient of thermal expansion for brass of 19.9 x 10^-6/K for brass and 16.3 x 10^-6/K for SS.
The Index Arm
The Index mirror is attached to the index mirror support plate by brass screws and on to the brass index arm. Once again, the thermal expansion of the arm will keep the mirror at the same angle relative to the pivot regardless of temperature, so no errors are introduced. The only thermal error introduced is the use of the 16swg stainless steel mirror. The difference in expansion expansion over 0.064" between brass and SS will cause an error of 0.000058mm for every 10C.
The Vernier
As the vernier expands and contracts, it's circular design is unaffected by changes in temperature. Changes in the radius of the index arm are counteracted by changes in the radius of the gear teeth. The thread pitch of the gear wheel may be affected a little, with the engagement of the thread in the frame's gear teeth increasing with decrease in temperature under the release spring load. This does not alter the angle reading by one iota.
Differences in thermal expansion rates between the silver steel vernier shaft and brass vernier assembly lead to an increase in axial clearance of 0.0019mm for every 10C reduction in temperature (may bind at high temps). 1 minute of arc is equivalent to 0.025mm, so a 140 metre positional error may be introduced by every 10C reduction in temperature.
No temperature effects relate to any other component with any effect on accuracy.
The total combined error caused by the index mirror error and the horizon mirror error equates to an error of 0.0001mm in the positioning of the parallel rays from the sun at a 60 degree apparent solar altitude (increasing to 0.00014mm at 90 degrees, reducing to 0.000058mm at 0 degrees). This possible error is vastly smaller than any machining accuracy you could hope for during manufacture of the sextant, so temperature considerations from this source may be ignored.
If you are really concerned about temperature induced error, I suggect you take the following precautions:-
1) Make the vernier shaft from brass.
2) Zero the sextant in the climate that you expect to use it in.
3) Every time you take a reading, ensure that there is no end slop on the vernier shaft.
(The last precaution is for the terminally cautious..!) By taking these precautions any small sextant error due to temperature should have been eliminated.
And that's it. It's not too difficult is it?
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