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    Sextant Design
    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 U.K.



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