10 THE MARINE SEXTANT
39. PARTS OF A SEXTANT
Fig. 10-2 shows a modern micrometer sextant of traditional metal construction in which the parts have been lettered to assist in the explanation, which follows;
Each sextant consists of a metal frame F, the lower curved edge of which is called the limb of the sextant. The limb is really the whole of the curved circular portion of the sextant (C in fig. 10-2) into which the graduated arc is bedded. At the back of the frame is fixed the wooden, bakelite or plastic handle or grip (B) of the instrument which enables it to be held in any position between the vertical and the horizontal, The arc (C), on which the graduations are cut, is part of the true arc of a circle and made of metal with a low coefficient of expansion. It is usually made of a thin piece of silver or platinum let in flush with the face of the sextant limb, and graduated in degrees from the right to left, from 0º (zero point) to 120º and on some sextants to 130º, 135º or 140º). To the right of 0º or zero on the arc is graduated 5º (sometimes 10º), which is said to be off the arc, and is called the arc of excess. Readings to the left or right of 0º are therefore distinguished as being on or off the arc respectively.
The index bar (D) is a movable arm, which is free to rotate on a central axis around a point on the frame, which is the centre or the circle of which the arc is a section. The index glass (I) is a reflector or mirror set in a brass frame perpendicular to the plane of the instrument and rigidly fixed to the index bar at its pivoting point, so that the index glass moves with the index bar and changes it direction as the direction of the index bar is changed. The glass is silvered all over and is designed to reflect the image of objects being measured upon the horizon glass, whence it is re-reflected to the eye of the observer via the telescope. In fig 10-2, the first adjustable screw (J) can be seen behind the Index Glass (described later).
The horizon glass (H) is set in a small metal frame rigidly fixed to the frame of the sextant perpendicular to the plane of the instrument and parallel to the index glass when the index bar is set at zero. One-half of the horizon glass (the half farthest away from the instrument frame) is plain glass and therefore transparent so that objects may be seen directly through it by the telescope; whilst the other half (nearest the frame) is a silvered reflector or mirror. This mirror receives the image of the object reflected from the index glass and transmits them to the eye of the observer, through the telescope. Behind the horizon glass, M and N indicate the second and third adjustment screws, respectively.
Horizon Shades (O) of coloured glass (usually three) of different densities are fitted to be turned up as required beyond the transparent part of the horizon glass to reduce excessive glare on the horizon from the Sun or Moon. Index Shades (K) of coloured glass (usually four) of different densities are fitted to be turned down as required between the index and horizon glasses in order to reduce the brightness of the Sun or Noon when observing these bodies.
The index bar can be set to any position on the arc AC by means of the releasing clamp (F) beneath it. When clamped, it can be given a slow motion to one side or the other by means of the micrometer drum (G), or in the case of a vernier sextant, by the tangent screw (located in the same position). The second’s vernier (H) is associated with the micrometer drum.
The arrow or zero of the index bar (called the index) is situated in the centre of the cut-away viewing space in the index bar on the micrometer sextant (E in fig 10-2), but to the right hand side (at 0º on the vernier) of the viewing space in the case of a vernier sextant.
A telescope collar (Q) is provided so that the sextant telescope (P) may be screwed into it and thus kept rigidly in the correct position. The collar can be raised or lowered by a milled head called a rising piece. The normal position of the telescope is where equal parts of the plain and silvered parts of the horizon glass are visible, but by raising or lowering the rising piece the telescope in its collar can be directed to either the silvered or plain glass portions of the horizon glass and the brilliance of the reflected image therefore regulated. As the telescope is raised, less of the silvered part of the horizon mirror appears in the field of view and the reflected image will not be so bright. If the telescope is lowered, however (that is screwed hard down), the brilliance of the reflected image will be at its greatest.
40. SEXTANT TELESCOPES
By enlarging the object observed the telescope makes accurate observation easier. The tube or draw of the telescope slides in or out to suit different eyesights. The sextant is usually provided with two telescopes.
The telescope of lowest magnification is the short conical bell-Shaped telescope and is used for all general observations. This is a Galilean telescope in which the image observed is seen the correct way up erect and is thus known to seamen as the Erect or Star telescope.
It has a large object glass (providing a large field of view) and a low magnifying power (providing a very bright image), these two properties are necessary for star observations for which reason the erect telescope is known as the star telescope.
The other inverting telescope is useful for making the adjustments of the sextant (described later) but, because of its small field of view, requires a steady platform for observation. As this is rarely attainable in a small vessel, the erect or star telescope is usually used for all observations.
41. BUYING A SEXTANT
As will have been gathered from the foregoing pages, a sextant is a most important and somewhat delicate Instrument, requiring great personal care and attention. Although it is not essential for the small craft coastal navigator to possess a sextant, the instrument can be of value in coastal navigation. For long-distance passages out of sight of land, however, a sextant is essential for obtaining fixes by Astronomical observations whether or not satellite fixing is available.
In buying a sextant, therefore, many factors will have to be taken into consideration, not the least of which is the use to which it will be put. For occasional use on weekend coastal navigation cruises, a high-priced traditional sextant may not be worth its cost and such a navigator might do very well with the low-cost plastic sextant. For frequent coastal navigation, the navigator might consider the precision accuracy weight of a traditional sextant well worth the high cost, but on the other hand he, too, may settle for the low-cost plastic sextant. For Astronomical navigation, there are more obvious advantages in the traditional sextant, but as already stated the low-cost plastic sextant has proved reasonably accurate and quite satisfactory for this purpose. The main advantages of the more expensive instruments are that they are larger, heavier, more accurate, and easier to read and have finer optics and accessories. In the long run, as with most things, you get what you pay for. A well priced instrument will last a lifetime if you take care of It.
In buying a new traditional sextant, the maker’s name will be sufficient guarantee of the quality and accuracy of its construction, a few of the best-known names: Browne, Cooke, Heath, Tamaya and Plath and Zeiss. Ebco and Davis are well known as reliable makers of plastic sextants.
A second-hand instrument should only be done from a reputable nautical instrument dealer. If possible get a professional seaman to help you select the best instrument. A second-hand instrument offered by a reputable dealer is unlikely to cost less than £150 and may be suspect if it does. On no account buy a second-hand sextant because it looks all right. Many a dirty-looking sextant covered with verdigris will; by spending a few pence to have it cleaned, be made to look far better and may turn out to be an excellent instrument, whilst the good-looking sextant may have a flaw which cannot be detected easily.
In selecting a second-hand instrument, several points should be watched. Ensure the glasses in the shades are not slack. Reverse the instrument and see if the true and reflected arcs are in the same continuous line, Note whether the arc is well cut, as this is often worn by constant polishing; if it is, reject the sextant at once. See that the adjusting screw at the back of the index (top) glass is workable, and examine the other screws for the same reason, because sometimes the head of a screw is so worn that a screwdriver will not grip it. Make sure also that the mirrors are not slack; bad silvering alone does not matter as they can be re-silvered at a marine opticians for a small sum.
Fig. 10-2, shown a traditional metal-constructed micrometer sextant of high quality and accuracy, which nowadays will cost around £1600.
Fig. 10-3 shows a modern yachtsman’s micrometer sextant of stabilised Makrolon, which is a high-temperature thermoplastic polycarbonate. The Mk 25 features a full field dielectric beam converger (which replaces the conventional half-silvered mirror) and combines the horizon and celestial images into a single field for easy, reliable sights under the most difficult conditions.
The beam converger is protected from salt spray damage by a special quartz-hard coat. A special mineral coating, which is selective for different wavelengths of light, transmits the horizon in one colour region and the star or sun in another.
Enhanced-contrast allows easier sighting at dawn or twilight and during daytime haze. An LED (light emitting diode) illuminating system floods arc and drum with soft light for night use. Arc range –2º to 127º, Scope 3x27mm, Mirrors, Horizon 50x38mm, Index 41x29mm, Shades Horizon, 3 plastic, Index 4 plastic, weight 16.5 ounces. Price around £120.00.
To prevent the sextant being jarred through being fitted loosely in its box, It Is usually held firmly in position by an arm or catch of some sorts and frequently this has to be released by a push or pull button or arm. Before attempting to remove any sextant from its box, an examination should be made of the exact arrangement of its fitting and securing.
Having released any securing fitting with the right hand, the sextant should be grasped firmly by the fingers around the framework about its point of balance and lifted from the box by the left hand, raised up and immediately transferred to the right hand, which grasps the handle or grip at the back of the instrument. Thereafter the sextant should be held inv the right hand and all adjustments to the parts of the sextant made with the left hand.
If it is required to place the sextant down at any time other than in its box, this should only be done on a flat surface. It is set down by grasping the frame of the instrument with the left hand, and placing it down on the sextant’s three legs with the handle underneath. The greatest care must be taken when lifting the instrument up or placing it down, to grasp the sextant firmly by the frame only, and never, to lift it by the limb or arc, index bar, mirrors, shades or telescope, because by so doing it may be strained.
Before returning a sextant to its box, it is essential to fold in the index and horizon shades towards the middle of the instrument. If they are left in the position for use, the sextant will not fit into the box and the shades may be knocked and damaged. Many sextant boxes are not large enough to take the instrument at whatever position on the arc at which the movable index bar may be left, and frequently it is necessary to move the index bar some way along the arc to allow the box to close. In this case, the ideal position is halfway along the arc.
43. SEXTANT CERTIFICATES
Fitted inside the lid of the sextant box or case there is usually the sextant certificate. In the cheaper models of sextant, this is issued by the makers of the instrument. The more expensive types of sextants, however are sent to the National Physical Laboratory for independent testing. If it is found to be up to a certain standard the N.P.L. issue a certificate of examination for that particular sextant, which is then permanently fitted in the sextant case.
Two classes of the certificate are issued, class A and class B, according to the standard of the instrument. The hallmark of a good sextant is to have an N.P.L. Class A Certificate. After examination at the N.P.L., all the non-adjustable errors of the sextant are tabulated on the certificate. For an N.P.L. Class A Certificate, these errors are tabulated for every 15º of the arc and must not exceed 40″.
For an N.P.L. Class B certificate, these errors are tabulated for every 30º of the arc and must not exceed 2′. If the errors exceed 2′ the N.P.L. will not issue a certificate.
Today, many sextants have the maker’s own certificate, this being perfectly satisfactory, and may be relied upon implicitly, but a keen navigator buying a new sextant is recommended to obtain an N.P.L. Certificate if possible. It is important that the maker’s name and the number of the sextant are both shown on the sextant certificate and engraved on the sextant itself, because if the instrument Is sent for repairs at any time it is readily traced by this number and make.