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    Re: An "alternative" sextant?
    From: Scott Owen
    Date: 2009 Jan 06, 01:04 -0600

    George Huxtable wrote:
    > Thanks to Scott Owen for taking an interest in my proposal for an 
    > "alternative" sextant design. He has made some perceptive comments, but  I 
    > need to explain the layout more clearly. Then I would like to hear what he 
    > has to say next.
    
    George,
    Thanks for the picture that did indeed clear things up quite a bit, but
    I still have a few questions and comments.
    
    > Drawing is not my forte, as will be clear from the attached sketch, which is 
    > intended to show the mirror positions and light paths, nothing more. HM and 
    > IM are horizon and index mirrors. There's a view from the observer's left 
    > and another from above his head.
    
    It took a minute or two of study but I think I at least got the 45deg
    angling change, location of mirrors and telescope clearer in my mind.
    
    > With this layout, a standard sextant arc would allow measurement only to 
    > 60�. If the mechanism was redesigned to allow the platform of the index 
    > mirror to be rotated by 180� about the pivot axis, as shown by dotted lines, 
    > then viewing the horizon astern would be possible.
    >  
    > I will copy my original message, with Scott's interpolations marked between 
    > parallel lines,  and with my own responses shown between {{ brackets like 
    > this}} with the letters G and S to clarify what's from me and what's from 
    > Scott.
    
    I shall only include Georges comments wherein I still have a question or
    comment.
    
    > Look at the instrument face-on, with its frame vertical. Your eye is placed 
    > on the pivot axis, looking horizontally. The index mirror is fixed to the 
    > index arm, centred on its pivot axis, but now twisted through 45� so that it 
    > reflects the incoming light towards your eye, along the horizontal line of 
    > that pivot axis. Undo the horizon mirror from its normal location and move 
    > it bodily up and right, a few centimetres , so that it intercepts your view 
    > (along the pivot axis) of the index mirror and it, too, is centred on the 
    > pivot axis. Of course, it has to be moved towards your eye a bit, away from 
    > the frame, so that the mirrors don't clash, and space is left to insert the 
    > index shades between the mirrors, and maybe a bit extra. It's now fixed to 
    > the frame there on a new bracket; re-angled so the horizon mirror is now in 
    > a vertical plane, but twisted about a vertical axis until its exactly 45� 
    > from the plane of the frame. Now light travelling along the pivot axis is 
    > reflected by the horizon mirror towards the right, still travelling 
    > horizontally. That's where you relocate the telescope, to collect that 
    > light, and also, as normal, to have a view of the horizon, alongside or 
    > through the horizon glass. So now, the telescope is in the same horizontal 
    > plane as the pivot axis: not below it, as before..
    
    The above "newer" description, is a lot clearer for me especially after
    looking at the picture.
    
    > However, there's another "essential" feature of the normal sextant that
    > will no longer apply. Normally, as the angle of the index mirror
    > changes, the deflection of light changes by twice that angle. But that
    > law is only true when all light paths are kept in the same plane. With
    > this alternative instrument, the light paths are in very different
    > planes (except at the zero-check position), such that the deflection of
    > light becomes equal to, not double, the angling of the mirrors. (If
    > anyone can offer disproof, please do so).
    
    With this new configuration you have definitely changed the plane but is
    it not a parallel plane?  Since it is a parallel plane would not the
    deflection angles still be twice the angle?
    
    > What I've explained, so far (with the aim of simplifying the picture) is how 
    > you could adapt an existing sextant to cope with the different geometry, but 
    > that world work only for angles up to 60�. For angles greater than 60�, 
    > however, and especially if extending the range to180�, mechanical redesign 
    > becomes necessary, and with the arc-radius reduced to (say) 9cm, radical 
    > redesign become very possible, indeed rather easy. For example, there could 
    > be advantages in having the drum and worm in a fixed position on the frame, 
    > and the (semicircular) arc moving with (or replacing) the index arm, rather 
    > than vice versa as at present.}}
    
    And indeed this is why I asked the question of where the index zero
    would be, 6o'clock position or nearer the eye at 3 or 4 o'clock
    depending on how your proposed 180deg arc instrument worked.
    
    
    > There would be a few advantages, if only minor ones. Because each mirror
    > always reflects through 90�, there's no shrinkage in the view to a
    > letter-box shape, as you get with a sextant at large angles. That
    > implies you could use the instrument right up to 180�, wall-to-wall
    > between horizons. to measure dip, as long as one proviso has been met.
    > That is, that the sideways displacement between the two sight-lines is
    > enough to allow incoming light from astern to miss the ear of even the
    > most jug-eared navigator. That's why there has to be a certain minimum
    > spacing between the two mirrors.
    > I haven't yet discovered any major drawbacks to such a redesign, and
    > hope that Navlist members will point out any there may be. For example,
    > is its calibration unduly sensitive to the exactness of those 45�
    > twists? Drawbacks there must be, I presume, if in all the years of
    
    Wouldn't the 45deg twist angle shrink the "side to side" view through
    the telescope just based on the viewable area of the mirror when set to
    45deg angles?  Wider mirrors?
    
    > The era of the sextant has been and gone; now is not the moment to be
    > proposing a redesign, and that's not what I am doing. I am just asking
    
    Why not propose a redesign especially if it's a better mouse trap?  Of
    course, it probably is not commercially viable today but that is of no
    consequence.
    
    > If no serious objections emerge to the principles of what's been
    > suggested, I can proceed to the practical details of how such a
    > very-different instrument might have been constructed, which I've been
    > pondering on, a bit.
    
    I'd be interested in those details if you've a mind to share them.
    
    -Scott
    
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