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    Re: Octant as dipmeter
    From: Alexandre Eremenko
    Date: 2004 Nov 20, 10:21 -0500

    Dear George,
    On your request, I include a complete citation
    of the relevant place in Norie (he refers to
    notation on the picture of the quadrant that you see):
    IV. To set the Back Horizon Glass perpendicular to the Plane
    of the Index Glass, 0 on the Nonius being at 0 on the Arch.
    Let the 0 on the Nonius be put as much to the right of 0
    on the Arch as twice the dip (taken from Table V.) amounts to:
    hold the Quadrant in a vertical position, and apply the eye
    to the back Sight Vane L; then if the reflected Horizon,
    which will appear inverted or upside down, coincide with that seen
    direct, the glass is adjusted;
    otherwise the screw in the middle of the lever on the
    under side of the Quadrant must be slackened,
    and the nut at its extremity turned till
    both horizons coincide.
    My comments.
    1. How the back sights are taken in general.
    Suppose the quadrant is oriented like on Norie's picture.
    You hold it with your left hand by the frame,
    using your right hand to move the arm, and look
    through the sighting vane L at the mirror H. This mirror
    has a slit that permits you to see the Horizon in front of
    you directly (you are looking horizointally left to right
    with respect to the Norie's picture).
    The celestial body behind you is reflected in the index
    mirror F first, then the reflected ray goes
    to the mirror H and reflected to your eye through the Vane L.
    Almost as in the usual sextant, except that the index mirror
    "looks back".
    2. Now, setting the arm near zero, you see the horizon behind you
    through two mirrors, and the horizon in front of you directly
    through the slit in H.
    And the sextant measures twice the angle between the
    directions at these two horizons;
    exactly what a dipmeter is expected to do.
    On your question, how to determine index error
    of the Back sights, one answer is to take known
    star-to-star distances.
    On Sat, 20 Nov 2004, George Huxtable wrote:
    > On my screen there's no such box or such an option to click on.
    > Instead, I
    > get a nice engraving of the Hadley quadrant, etc.,
    > but otherwise am stuck
    > within the 8 pages of the preface, with no apparent means of escape.
    Try entering this URL:
    This brings to directly to the beginning of the chapter on
    Hadley quadrant, what I cited is on p. 6 of this chapter.
    > I suspect that for most modern sextants there
    > is insufficient headroom even
    > for that.
    My further experiments show that it could be "just enough".
    The extra mirror has to be attached near the very left
    end of the arc. And of course, the mirror frame can be designed
    so that it extends the sextant frame a little bit down.
    Of course I have no intention to drill a hole in the frame of
    my cherished SNO-T (not speaking of my forhead), but sometimes
    I can buy a plastic sextant or an old SNO-M for such experiments,
    by the way the frame of Davis Mk 15 sold by Celestaire
    is by 16 mm larger than that of the modern metal sextants.

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