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    Sextant and quintant limitst: was: sextants on aeroplanes
    From: George Huxtable
    Date: 2008 Dec 14, 10:41 -0000

    Gary LaPook [6732] and Bill Morris [6734] have both added insights about the 
    upper limit of angle that can be measured by sextants (or quintants).
    Some of what follows has been said before, on this list or its predecessor, 
    but it may bear repeating.
    When the horizon view-line of a sextant is, as usual, set to horizontal, 
    objects close to the direction of the line joining the two mirrors would be 
    nearly edge-on to the index mirror, and the view in that mirror would have 
    shrunk to a letter-box slot of almost nothing. So the slope of that line is 
    the limiting factor, depending on how narrow a slot the observer is willing 
    to tolerate. There's no point in extending the calibrated arc further than 
    that practical limit, though for many sextants, that's (pointlessly) the 
    case. That slope is governed by the design of the frame; the nearer the line 
    between the mirrors is to the horizontal, the greater the range of angles 
    that can be observed. The tilt of the horizon mirror must, of course, be 
    preset to correspond.
    To reduce the slope of the line between the mirrors requires, first, 
    minimising the vertical offset between the telescope centre-line and the 
    pivot-axis of the index arm. There's a practical lower limit, depending on 
    the aperture of the telescope, which still allows an unobstructed light path 
    with room for the index shades. Having done that, the next step is to shift 
    the horizon mirror further away from the observer. And that's the feature 
    that usually distinguishes a quintant, for which, to earn its name, it 
    should be practical to measure angles to at least 144�. The horizon mirror 
    is often mounted on a small outrigger, so that it's offset; still centred in 
    the telescope view-line, of course, but now further away along that line.
    You can often identify a proper quintant, at a glance, just by the 
    positioning of the horizon mirror. But don't be misled into judging it by 
    the number of degrees divided on the arc. Find the angle setting, near 
    maximum, at which the mirror-view, though shrunk, is still usable. That's 
    what matters. Of course, the divisions must extend at least that far.
    However, it seems to me at least plausible for a 2-mirror instrument (which 
    couldn't, any longer, be called a sextant) to be designed, which could 
    measure angles from zero to 180�, though nobody ever seems to have done so. 
    That's material for another posting.
    contact George Huxtable, now at george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 
    Navigation List archive: www.fer3.com/arc
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