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 Rodney Myrvaagnes wrote:

> Has anyone tried making a prismatic horizon that dangles like a damped
>pendulum?

Yes, I used a Davis plastic sextant as the basis for a novel (as far as I
know) design of sextant incorporating an "artificial horizon" mirror. This
mirror was balanced on a single point in such a way that it remained in the
horizontal plane regardless of any modest tilt or rocking of the sextant.
The movement of the mirror was damped by virtue of a copper strip attached
to the mirror mount. As the mirror tilted, the copper strip moved in the
field of a small rare earth magnet fixed to the sextant frame, and so
damped the movement.

The way the sextant worked was as follows. The image from the index mirror
was reflected down through an image inverting dove prism, to a partially
reflecting mirror in place of the usual horizon mirror. At this point, part
of the image was reflected back to the telescope in the usual way, and part
of the image went through the mirror, where it was reflected off the
"artificial horizon mirror". Another fixed, fully reflecting mirror then
reflected the image coming up from the horizon mirror back through the
partially reflecting mirror and into the telescope.

In this way, two images of the same object were seen in through the
telescope. The sextant was rocked sideways until both images were in the
same vertical plane and the angle of the index arm was adjusted until the
two images were superimposed (for stars) or touched in the usual way for
sun or moon. The virtue of this arrangement was that image was invariant to
sextant tilt, just as with a normal sextant, and no other illumination was
required as with a bubble sextant.

The sextant worked quite well, but the artificial horizon mirror mount was
difficult to make. I never did manage to make the mirror mount so that the
mirror stayed perfectly horizontal as the sextant was tilted. Though the
principle was proven, I did not see any way to overcome this problem in any
simple way, so I gave up on this project. I did try a small bowl of mercury
as a horizon mirror, but the mercury would not keep still! The slightest
knock or movement would send shivering waves across the surface of the
mercury.

Geoffrey Kolbe.



Dr Geoffrey Kolbe, author of "Long Term Almanac 2000-2050" for sun and
selected stars, with concise sight reduction tables.
Available online from www.pisces-press.com

   

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