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    Re: Finding IC for A-12 bubble sextants
    From: Bill Morris
    Date: 2021 Oct 5, 14:29 -0700

    It is very helpful when making adjustments to have the instrument mounted firmly. A stout photographer’s tripod is ideal, as the screw will fit the tapped hole just behind and above the bubble unit . You will need to interpose a spacer or stack of washers so that the tripod does not foul the bubble unit. If using a horizontal collimator, set the instrument to zero. If using a horizon, you will have to set the index to below zero by the amount of dip, so that when you set the reading to zero, the line of sight through the bubble will be truly horizontal.

    Loosen the locking screw on the mirror hub so that it is possible to rotate the mirror into position, while leaving enough friction for it to stay where it is put. Fold the shades forward out of the line of sight and, viewing through the eyepiece, rotate the instrument about a horizontal axis until the bubble is centred. Then rotate the mirror until your horizontal standard bisects the bubble. Tighten the locking screw and check that the horizon still bisects the bubble and that the index reading is still where you set it. You may have to make several attempts, as it is quite difficult to tighten the screw without disturbing the mirror.

    For more details, see my reconditioning manual. Details at www.sextantbook.com

    Bill Morris
    New Zealand

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