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    Re: Plath Sextant: Advice - Required.
    From: George Huxtable
    Date: 2004 Jan 17, 12:35 +0000

    In reply to Kieran Kelly's posting, copied below-
    I am no sextant expert. All I have ever owned have been of plastic, though
    I have often used  metal sextants belonging to others.
    However, I can offer a bit of secondhand information, from Charles H
    Cotter's "A history of the navigator's sextant" (Brown, son, and Ferguson,
    Glasgow, 1983.
    He provides some references which may be useful, but which I have not seen.
    Heath, G A (not dated but about 1935) "Modern Sextants", London.
    Hughes A J, (1938) "The book of the sextant", Glasgow.
    Cotter also refers to his own paper, "The sextant and Precision Celestial
    Navigation", in Journal of the Instite of Navigation, vol 16 p 246, London.
    Here is what Cotter has to say about some of the matters Kieran asks about,
    on pages 168 to 169-
    "The sextant is provided with a collar to accommodate a telescope or blank
    tube .. The collar is fitted at the end of a device known as a rising piece
    which permits the axis of the telescope to be adjusted relative to the
    plane of the instrument.
    The collar in early sextants comprised a double brass ring constructed so
    as to furnish a means of adjusting the axis of the telescope to bring it
    parallel with the frame of the instrument. It is important that the three
    parts of the zig-zag ray which emanates from an observed celestial body and
    which, after double reflection, enters the observer's eye, are in the same
    plane. If they are not, because either or both of the mirrors are not
    perpendicular to the plane of the instrument, or because the line of sight
    of the observer is not coincident with the axis of the telescope (assuming
    that it is parallel to the plane of the instrument), error might result.
    Any error due to the oberver's line of sight not being parallel to the
    plane of the instrument is known as collimation error.
    If collimation error exists due to the axis of the telescope not being
    properly aligned, and if the sextant is designed so that the error can be
    eliminated, two small screws working in the double brass ring of the collar
    are turned, one is slackened before the other is tightened. This action
    slews the axis of the inner of the two rings relative to the plane of the
    instrument. But most modern sextants are not fitted with the means for this
    adjustment, the sextant maker having ensured that the axis of the collar is
    correctly aligned. But collimation error may result when the observer is
    careless in his observation. It is important that the line of sight is
    parallel to the plane of the instrument, and this may not always be the
    case when observing. It is important that whwn taking an altitude that the
    images of the observed body and the spot on the horizon vertically below it
    are exactly at the centre of the field of view of the telescope. A note on
    this matter is given be the writer"
    (GH adds- that note is the paper by Cotter referred to above. It may be
    that one use of the four alignment wires that Kieran mentions may be in
    aiding that centring.)
    "The purpose of the rising piece is to alter the relative amounts of light
    entering the observer's eye from the celestial body and the horizon
    respectively. By turning the milled head screw on the rising piece the
    telescope can be 'raised' or 'lowered' with respect to the plane of the
    instrument. By raising the axis the telescope is directed more and more to
    the unsilvered part of the horizon glass, thus reducing the amount of light
    entering the telescope tibe (and hence to the observer's eye) from the
    silvered part of the horizon mirror. For star observations the rising piece
    should be adjusted to allow the maximum amount of light to enter the
    telescope tube from the star after its image has been doubly reflected. For
    Sun sights and terrestrial observations the rising piece should be adjusted
    so that it is at the mid-position. Any reduction of sunlight entering the
    telescope tube is best achieved by using appropriate shades."
    I can find no mention in Cotter of a binocular attachment, but I can
    imagine it might be very useful in finding a particular star, because its
    feld of view in the left eye would presumably be much wider than the field
    of view in the right eye, which is limited by the small solid-angle of the
    index mirror of those earlier days. It would presumably allow the observer
    to see a whole constellation.
    Kieran Kelly wrote-
    >I have recently added a Plath sextant - circa 1920 - to my collection and I
    >would appreciate some advice from list members regarding its features.
    >1. RISING PIECE The sextant is fitted with a rising piece which moves the
    >telescope in or out while maintaining its parallelism with the frame of the
    >instrument. This is designed to allow more of less of the horizon/ celestial
    >body in the telescope. Why was this necessary? Why not just have the
    >telescope a fixed distance from the instrument and which bisects the horizon
    >mirror as in modern sextants. Is it because the horizon mirrors were so
    >small in days gone by?
    >2. COLLIMATION RING The sextant is fitted with a collimation ring which
    >allows adjustment to the axis of the telescope bringing it into exact
    >parallelism with the instrument. Why was this necessary? The amount of error
    >in the observation produced by collimation error must have been very small,
    >as the telescope is not prima facie a measuring device. One suggestion I
    >could make is that the Plath sextant in question can be completely taken to
    >pieces, down to is constituent parts and then reassembled, making it very
    >convenient to clean and service. However the flip side of this is that it
    >must be completely readjusted when re-assembled.
    >I wonder why the rising piece/collimation ring system was abandoned? Its
    >great advantage is that if the sextant telescope  mount is given a hard
    >knock it can easily be re-calibrated by the observer. On a modern Plath the
    >telescope post is fixed and if it was bent, or worse still broken, the
    >sextant would be ruined. With the old models if the mount/rising price was
    >bent you simply bought a new one. This seems very sensible to me.
    >3. INVERTING SCOPE      The sextant comes with an inverting telescope with 4
    >wires for making collimation adjustments. I have looked up Oswald M Watts'
    >excellent "The Sextant Simplified" for instruction on using this scope for
    >collimation error, but would appreciate advice on other books dealing with
    >the subject. Is anyone aware of a good book circa 1900-1925 that  deals with
    >the care and adjustment of sextants?
    >4. PLATH SERIAL NUMBERS The sextant has no papers but its serial number is
    >8368. I know that by end of 1925 the Plath numbers had reached 10,500. I
    >presume they made about 400 items per year so that 8368 indicates date of
    >production about 1920 or not long after the First World War. Could anyone
    >help me out here?
    >Does anyone else on the list own a Plath in the 8000 numbers?
    >5. MIRRORS The sextant in question has rear surfaced mirrors. Does anyone
    >know when Plath went to front surfaced mirrors or when manufacturers in
    >general went to front surfaced mirrors? It also has the very small, square
    >horizon mirror. Does anyone know when Plath went to its now typical large
    >horizon mirrors?
    >6. BINOCULARS The sextant came with a small pair of binoculars which had a
    >post for fitting to the rising piece. I cannot for the life of me imagine
    >how a binocular could improve sights over the monocular telescope, although
    >Watts says that many sextants came with binoculars in addition to the
    >standard telescope monocular, although they were not popular with seamen. He
    >doesn't say what they were used for.
    >Any help would be greatly appreciated.
    >Kieran Kelly
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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