NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: JC Sutherland
Date: 2003 Mar 4, 23:39 -0000
Hi all;
The shape and construction of the Horizon mirror is an
important feature of a sextant and each manufacturer will have their own
designs. For example, some have a
full glass window half silvered and some have a mirror shape to cover only half
the aperture of the telescope. Some have front silvered glass; some have this
with a cover glass to protect the mirror, while some back silver the glass with
a layer of paint over it. It is very difficult to cover the silvering (or should
I say aluminizing) with a layer of paint that can stop tarnishing at the edge
without it continuing past the edge to show as a black line in the field of
view and this may account for by Fred Hibbard’s problem. Several glass
surfaces tilted as they are at this mirror would also create the multiple
images he mentioned. It aggravates the problem that the mirror has to be thick
enough not to bend with the action of the adjusting screws.
It is possible that the design of this mirror depends on
whether the instrument is optimised for sun or for star sights. Maybe there is
an opinion on this?
In some sextants the Filters are big enough to cover the
full aperture of the telescope while some others will let light past them into
the field of view. The sea horizon glint beneath the sun will put a lot of
stray light in the eye but it is often the Index filters that are the problem
as they are further from the telescope.
If the sextant telescope has a rise and fall mount, this can
be used to adjust the relative brightness of the sun in either half of the
horizon mirror. I don’t know if this proper use for it. Perhaps someone
on the list can put me right.
For my preference, I would like to see a horizon mirror
frame big enough to stop stray light fitted with a half mirror glass i.e no glass on the horizon side. With a knife edge mirror,
front silvered with a Chromium coating that does not tarnish. Better still,
using ground and polished Chrome flats instead of fragile glass.
My sextant is a Tamaya and I like
the weight of the metal frame but it has its shortcomings. I can generally get
position lines with less than half a mile error, even on a small 30 ft boat,
provided the sea is calm, but I have an EBBCO plastic sextant that I use when
precision is not necessary. I think it is better to learn on a good instrument
as you don’t want extraneous problems while you are struggling to learn the
basics.
I don’t
think I like George’s trick of standing on his to flatten it after it had
warped. ;>)
An off topic curiosity I have come across, is having the
Index error set to 1 deg instead of zero. I suppose this is to make sure that
the IE is never negative. (A 1 deg error would be obvious and small variations
would never reverse the sign). Some users have a difficulty reading IE,
‘off the arc’. This was especially so using the RAF Mk IX bubble
sextants and the method was often suggested by Navigation Instructors. I have
not come across it being applied to marine sextants however. Perhaps someone
else has?
Clive. Sutherland
Wantage