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    Re: Dip Anomaly
    From: Clive Sutherland
    Date: 2008 Mar 14, 01:41 -0000
    Over the years several people have modified the Mk IX bubble sextant to allow it to be used as a marine sextant.
    My own version comprises removing the clockwork and drilling a hole through the case in line with the view from the eye , through where the  5 deg mirror goes. This is then replaced with a half silvered mirror.
    With this arrangement it is possible to see the horizon at the same time as the bubble. However the sun filters have to be removed and other means provided of reducing the light from the sun.
    An alternative design that I considered was to remove the window of the  illumination aperture at the top of the sextant, where the hook is fixed, and mounting a good 90 deg Pentaprism looking forward, in its place,. A simple mirror here won't do as this would need an elaborate calibration procedure to set it up. As it is, quite a lot of bodging is needed to get it right. but the beauty of this design is that the averaging clockwork would still be available as would be the filters.
    I also have an article somewhere about a modification being done, with a prism fitted beneath the sextant to convert  the Mk IX completely into a marine sextant.  The reflecting mirror and lens of the bubble mechanism is  completely removed and a hole cut in the bottom of the case against which the pentaprism is mounted . Of course the bubble is now unusable and this modified sextant could not be used to measure Dip
    The best arrangement for measuring Dip would be to use a "Bish sextant" that observes the horizon both in front of and behind the observer simultaneously. George Huxtable investigated this method via the list some years ago so it should be in the archives.
    I would be very interested to hear of any other ideas that may have been tried.
    ----- Original Message -----
    From: Peter Fogg
    Sent: Thursday, March 13, 2008 10:50 PM
    Subject: [NavList 4666] Re: Dip Anomaly

    Greg Rudzinski asks:
    "Can a bubble sextant be used to measure dip
    anomalies from a known height of eye?"
    The proposition implies using both a conventional and bubble sextant consecutively and comparing the results.
    I guess one problem is that bubble sextants do not tend to give particularly precise results.  They are also notoriously difficult to use (to hold level) on-board. 

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