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Bill Morris wrote a perceptive note, copied below, about the Observator
sextant.

And the patent is just as he describes it. It's true that the two filters do
indeed "lie behind the objective", but so far behind it that they are
closely in front of the Galilean eyepiece. There, they sit side by side, the
horizon shade to the left and the reflected-light shade to the right,
controlled by separate adjusting knobs.

I wonder if any listmember has ever tried one of these sextants. It would be
interesting to know how they behave. It seems to me that the placing of the
observer's eye, horizontally, with respect to the eyepiece, would be
crucial, in that the ratio of the light intensity of the two images would
depend greatly on that positioning, a bit like the adjustment of a
conventional sextant by a "rising piece" shifting the telescope. On the
other hand, that might actually be an advantage. Only experience, in use,
would tell.

Another possible difficulty might be that unshaded direct bright Sunlight
will enter the objective lens, and might scatter about within that lens to
affect the other view. If that happened, it wouldn't be cut out by the
internal obscuring screen, within the telescope.

A disadvantage, with star sights, is that both star and horizon views will
have the light-gathering halved by that obscuring screen; just as if the
aperture of the objective was reduced by root-2, or one stop.

Finally, because the filtering is done by  curved plastic screens, which are
most likely produced as an image on photographic colour film, checks could
be called for to ensure that there had been no degradation of the film, or
its dyes, over the years. Presumably, the sextant would have lived in a dark
box all its life, so should not have been affected by light-bleaching. But
those films may be less long-lived than the coloured glass shades used in a
conventional sextant.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

Members can read the original patent document at
http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=EP0082556&F=0&QPN=EP0082556.

Its claim to originality are that the filters are contained safely
within the viewing means and that they can be made of cheap material
like photographic film, as they do not have to have flat parallel
faces, lying as they do behind the objective lens of a Galilean
telescope. The index beam and the horizon beam do in fact have
separate filters which do not get "filthy" like ordinary ones, nor do
they invited destruction at the hands of the clumsy. The filter
density can be made continuously variable.

Bill Morris

On Aug 14, 9:25 am, "Richard B. Emerson" 
wrote:
> I've never seen this sextant before but I looked over the eBay auction
> and decided it has one big problem: filters. As best I can tell there's
> no way to use one level of filtering for the index mirror and another
> for the horizon. Things like a bubble horizon are out (unless one's
> built in?) as are astigmatizers and varying scope powers. Of course,
> all off that could be built in. But I'm not holding my breath on any of
> that being present. Good luck with your bidding! :-)
>
> Rick Emerson
> S/V One With The Wind
>
>
>
> Greg Rudzinski wrote:
> > Has anyone on the list used an Obervator Mark 4 sextant (Dutch
> > made) and how does it compare to other sextants ? There is one listed
> > on Ebay right now.
>
> > Greg- Hide quoted text -
>
> - Show quoted text -



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