NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Dip Anomaly
From: Clive Sutherland
Date: 2008 Mar 14, 01:41 -0000
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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.
Clive.
----- Original Message -----From: Peter FoggTo: NavList@fer3.comSent: Thursday, March 13, 2008 10:50 PMSubject: [NavList 4666] Re: Dip AnomalyGreg 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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