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    Re: Suitable Sextants - Mirrors
    From: George Huxtable
    Date: 2005 Oct 14, 22:19 +0100

    Joel Jacobs wrote, about the advantages of bigger mirrors on a sextant-
    >What you and Frank are missing is that the viewer's eye is not always
    >centered in the ocular lens of the scope due to the motion of the vessel.
    >If it  were attached to the sextant as your comments presuppose, then what
    >you say would be correct. But it isn't. The sextant and the hand holding
    >it and the eye viewing are independent from one another. What the viewer
    >sees is not in the laboratory setting upon which your argument is based.
    >Nothing that you or Frank have offered changes my opinion.
    I don't see what that has to do with the matter. Certainly, to get all the
    light that's available, the pupil of the eye needs to be aligned with the
    centre of the eyepiece (and eyecups help in that regard).  Just clamp a
    telescope to look at some object, then scan the eye past the narrow beam of
    light that emerges from the eyepiece. Displacing the eye from the centre by
    a very few millimetres causes a sharp reduction in brightness, but in all
    other respects the image stays exactly as before. The field of view hasn't
    changed. For areas that are fully outside that field of view, it matters
    not whether they are occupied by mirror-frame or by glass; no light from
    those areas reaches the eye, whether it's on-axis or not.
    >Second, you say  "It seems important to contradict "Joel's method" for
    >taking sights; otherwise novices might be tempted to follow it. Instead,
    >read any  textbook, and do the job the other way."
    >You also amuse me when you quote unnamed texts to prove a point, and will
    >declare later that you never do such things
    >George, here is a link to photographs of page 108, Figure 6-4 of Bauer's "
    >The Sextant Handbook". I would like you to publish some illustrtions from
    >other texts that show a different picture from that which I discribed. For
    >those interested, Bauer goes into great detail in the methodology of
    >swining an arc on pages 105-108 which are the same as my own.
    I don't have a copy of Bauer, but have taken a look at the upper sketch
    from page 108, which Joel copies. It illustrates my point precisely.
    There are three pictures, each showing a superimposed  image of the Sun
    (seen via the index mirror) and the horizon (seen through the
    horizon  mirror). For one, the sextant is upright; for the others, it's
    tilted about 40 degrees left abd 40 degrees right. Rather more tilt than we
    would usually use when rocking a sextant, perhaps, but there's nothing
    wrong with exaggerating for the sake of making a point.
    The sketch shows in each case that the Sun is visible in the field of view
    of the index mirror. Not always exactly at the centre of that field of
    view, its true, but no more than a degree away from that centre. How can an
    observer keep the Sun so well centred in the sextant view, while rocking
    the instrument about through such a large angle? Only by ensuring that the
    rocking is about an axis which is a line joining his index mirror to the
    Sun. That's exactly what I was advocating as the way the job ought to be done.
    I suspect that in real life, that's the way that Joel does the job too,
    perhaps without realising it. But it's not how he described it, when he wrote-
    >The procedure of rotating the sextant is to have the celestial body
    >subtend an arc in which its lowest point just kisses the horizon. At that
    >moment, the sextant is perpendicular, and the reading is taken. To do that
    >correctly, the object will sweep across the field of view as an arc, and
    >not be centered "at all times" as he suggests.
    It's the last sentence that's wrong. The sketch in Bauer shows the object
    centered "at all times", over a wide range of rock angles, and not
    "sweeping across the field of view as an arc".
    I don't know whether Bauer's description, in text, conforms with his own
    sketch or with Joel's explanation of how a sextant should be rocked. It
    might be interesting to learn.
    >Since I think you have a sincere interest in "knowledge" and enjoy sharing
    >your opinions in great detail with others,
    Thank you for the compliment, Joel ...
    >what I would like to learn from you is whether contemporary sextant's
    >mirror size and scopes are optimized. So you need not search for the
    >dimensions which are typical of most major brands they are:
    >Scope 4 x 40
    >Index mirror, 57 x 42mm
    >Horizon Mirror, 57mm dia.
    Happy to give my opinion on that matter, Joel. I would just like some
    further information first, please, about your "typical" sextant. What's
    your estimate of the field of view of the scope, in degrees? And what's
    your estimate of the path length, in mm, between the objective of the scope
    and the mid-line of the horizon mirror, and the distance between the
    mid-lines of the two mirrors?
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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