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    Re: Centring Error Detector
    From: Bill Morris
    Date: 2014 Jan 22, 23:40 -0800


    I think your "Ultradex" is my "face geared indexing table". The underside of the table and the top of the base have radial gear teeth that mesh with each other. It is the only appliance that I can think of that gets more accurate with wear, as any inequalities of the teeth are averaged out with wear. The makers take advantage of this by wearing it in by repeated meshings before sale. I made one once to prove that I could.

    A conventional way of using this to calibrate a sextant would be to place the sextant on the table, fairly well centred, though great precision is not needed, and to direct an autocollimator beam on to the index mirror. You would then rotate the mirror through, say, 7 1/2 degrees (15 on the sextant scale) and rotate the table the same nominal amount in the opposite direction, reading the error in the reflected beam off the autocollimator scale. This of course relies on the indexing table being accurate, but it can be calibrated using a mirror and two autocollimators to within the limits of the autocollimator (2 seconds in my hands but 0.5 seconds with lots of practice).

    Interested readers can see the layout and read a further description following Table 1 here: http://sextantbook.com/category/chasing-tenths-of-an-arcminute/
    Just imagine there's a rotary or indexing table instead of the modified SNO-T sextant underneath the sextant being calibrated.

    Brad, I think it would help understanding of your proposed method if you could show a simple sketch of the light path. I don't think I understand the role of the beam splitter.

    Bill Morris
    New Zealand

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