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    Re: Centring Error Detector
    From: Peter Monta
    Date: 2014 Jan 26, 18:26 -0800
    Hi Brad,

    I've been thinking a little about this---pardon me if I go off on a small tangent.

    Could this technique be used for sextant construction?  With these face-meshed gears one can make highly repeatable angle settings.  With several in cascade, one could have high resolution too.

    For example, imagine a 3D-printed plastic or metal plate with 360 ridges on the top surface, something like the lines of a protractor, but more extended radially.  (With a plate radius of 60 mm, the pitch will be 1 mm, quite doable with FDM 3D printers.) It will mesh with a similar network of ridges in an adjoining plate, and under a suitable preload and with kinematic averaging over such a large surface area, repeatability will be good.  Now put two of them in cascade, the second with 361 ridges/teeth.  Now the settability goes from 1 degree to 10 arcseconds, small enough that one could just set it close and wait for a touch of a celestial body.

    The index mirror would be mounted on the top plate, there would be a thin "idler" plate sandwiched in the middle, and the bottom plate would carry the horizon mirror and scope and shades as usual.

    To calibrate, make several sextants and run them against each other, perhaps under autocollimation with a camera doing the measurements.  With enough (random?) angle settings on each sextant, an error model could be estimated.  (Yes, that's pretty hand-wavey.)

    Could be better than a vernier plastic sextant, and cheaper too.  Could think about a hybrid construction with thin plastic or metal plates bonded onto a high-stability platform like carbon-fiber honeycomb.


    ps: best to "skew" the ridges so they're not radial but instead canted something like a herringbone gear, the better to smooth out any printer anisotropy.

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