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Oh, dear. I AM getting error-prone...

I wrote, in NavList 1588, about estimating the offset between the two
views from a sextant, when it's set to zero on the arc.
|
| Or you can do it by geometry. For that, you will need the effective
| distance d between the parallel reflecting surfaces of the two
| mirrors. You can measure this between the upper part of the horizon
| glass and the lower part of the index glass, where there will
usually
| be some overlap between those parallels. If they are front-silvered,
| that's easy. Otherwise, if you want to be precise, you will have to
| add two-thirds of the combined thickness of the two glasses; the
| two-thirds factor allowing for the refractive index of glass.
|
| And you need the angle A by which the horizon mirror is tilted from
| the plane that's at right angles to the collimation line of the
| telescope. You should be able to estimate this using some sort of
| protractor (or course plotter). Then the spacing between the
| sightlines is then d x cos 2A.

But it isn't, I'm afraid. It's d x sin 2A, if A is defined in the way
I had suggested. Sorry about that. I thought there was something
wrong, when comparing the offsets by the two different methods I had
proposed.

I have two very different sextants, a plastic Ebbco micrometer-type,
and a Vernier Heath-clone, and in both the offset angle A is pretty
near 20 degrees, by a rough visual check using a Douglas protractor;
perhaps a degree or so less. A better method may be to lay the sextant
flat down, photograph it from vertically above, and measure off the
photo with a protractor, but I haven't done that (yet).

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK..


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