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    Re: Sextant Arc Calibration : FIRST LIGHT
    From: Bill Morris
    Date: 2014 Mar 19, 23:15 -0700


    A few suggestions:

    a) You will need some means to set the plane of the sextant parallel to your surface plate or, more exactly, that the axis of the index arm and the plane of the index mirror is perpendicular to it.

    2) You will need some form of kinematic location so that the sextant stays exactly where it is put and can be replaced there, without introducing stresses into the frame, eg, one leg in a tapered (ideally pyramidal) hole, one in a vee groove and one on a plane. The sextant does not need to be precisely centred for this method.

    3) I had not appreciated that your Ultradex indexing table is so small. Have you considered adding a counter balance for your projected aluminium plate-plus-sextant?

    4) It would also be an idea to lay in a supply of plasticene to hold the bits where they are put on the surface plate. It is annoying to accidentally nudge something out of place in the middle of s long series of rather tedious measurements.

    5) My granite surface table rests on a layer of thick felt on a well-braced heavy timber and plywood base, so that the the base conforms to the surface table rather than vice-versa.

    I am sure you know all this, but other members may not appreciate the attention to detail necessary when making very fine measurements. See for example http://sextantbook.com/category/chasing-tenths-of-an-arcminute/

    Bill Morris
    New Zealand
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