12  SEXTANT ERRORS AND ADJUSTMENTS

Just as a compass must be adjusted, so must a Sextant – its very construction makes it quite a delicate instrument, and, bearing in mind the conditions in which it is housed, it may at any time receive some accidental damage. A light blow is sufficient to put it out of adjustment. The successful navigator is he who keeps his sextant in the best working order and in the highest state of adjustment. There are four main adjustable errors, which the navigator can control.

It is important that these adjustments are practised in the first instance under the supervision of an experienced navigator or nautical instrument maker, as considerable damage can result from misuse of the Adjustment Screws.

45. THE FIRST ADJUSTMENT – ERROR OF PERPENDICULARITY

This error arises when the index mirror is not perpendicular to the plane of the sextant. To check for this error, set the index to about 60º’ on the arc, then, holding the sextant horizontally with the arc away from you, glance obliquely into the index mirror keeping both eyes open and observe whether the reflected arc as seen in the mirror to the left is in the same plane as the arc seen by direct vision close outside the mirror to the right (in other words, are the arcs continuous in one straight line?).

If the true and reflected arc are continuous in one straight line, then the index glass is perpendicular to the plane of the instrument and no adjustment is necessary. If however, the arc appears broken where the images meet (i.e., the line of the reflected arc in the mirror is seen higher or lower than the actual arc as in fig. 12-1), then an error exists.

To adjust for Error of Perpendicularity, the First Adjustment Screw (at the back of the index glass) is gently turned (with the small bar pin provided in the sextant box) until the true and reflected arcs are in line again. It should be noted that sextants fitted with circular index glasses are permanently adjusted and no adjustment screw is fitted to such mirrors.

46. THE SECOND ADJUSTMENT – SIDE ERROR

This error arises when the Horizon Glass is not perpendicular to the plane of the sextant. To detect the presence of this error, set the index of the sextant to zero (0º00’00”) and, holding the sextant horizontally, or slightly tilted from the horizontal, points the telescope at the sea horizon. The true and reflected horizons should form a continuous line but if they do not (as in fig. 12-2), side error exists. This may be removed by gently turning the Second Adjustment Screw at the back of the horizon glass. There are two adjusting screws at the back of the horizon glass (and they may not always be fitted in the same position), but the Second Adjustment Screw is always the one in the centre line of the horizon glass (screw A in fig. 12-2). At night, the Second Adjustment may be carried out by holding the sextant vertically, the index at zero as before, and pointing the telescope at a star.  If side error exists two stars, reflected and true, will be seen side by side as in fig. 12-3 (a). These can be brought into coincidence by turning the Second Adjustment Screw as above.

47. THE THIRD ADJUSTMENT – INDEX ERROR (I.E.)

When the index is set at zero (0° 00′ 00″), the Index Mirror and the Horizon Glass should be exactly parallel to each other. If they are not, I.E. exists. To ascertain whether the sextant is free from I.E. or not, the sextant should be held vertically and, with the index set to zero, the telescope pointed at the horizon. If the true and reflected images of the horizon do not form a continuous line, I.E. exists and may be removed by gently turning the Third Adjustment Screw at the back of the horizon glass, (the one off the centre line).

Since the Second and Third Adjustment Screws both operate on the horizon glass, one adjustment tends to affect the other.

For this reason, the Second and Third Adjustments should preferably be made simultaneously using a star. Set the I.E. to zero and hold the sextant vertically, pointing the telescope at a star. If side error but no I.E. exists the true and reflected images of the star will appear as fig. 12-3 (a). If I.E. but no side error exists the images will appear as in fig. 12-3 (b).

If both side and I.E. exists, the image will appear as in fig. 12-3 (c) or (d). The two images are brought into coincidence by gently turning the Second and Third Adjustment Screws alternately. This procedure may have to be repeated two or three times before all errors are removed. If it is not possible to eradicate both errors the Side Error must be completely removed, and any residual left as I.E. The value of I.E. can be determined as described below and allowed for when taking observations.

48. FINDING THE INDEX ERROR.

If the I.E. is reasonably small, say up to 3′, it is generally preferable to find the error and then apply it as a correction to the observed sextant angle. Owing to the danger of the instrument being put out of adjustment by the adjustment screws working loose unknown to the observer, it cannot be too strongly emphasised that the screws should be used as little as possible. Constant adjustment is not advisable. ‘Tormenting’ a sextant is the term used for its ‘over-adjustment’.

The navigator will only check over the adjustments occasionally, but will constantly check I.E. When ever a sextant has I.E. this must be added to, or subtracted from, every angle measured with the instrument.

There are three ways of finding I.E.:

•   by observing a star
•   by observing the sea horizon
•   by observing the sun’s diameter ‘on’ and ‘off’  the arc.

The first and best way is to observe a star with a low altitude. With the index set approximately at zero (a few minutes one way or the other off zero), and the sextant held vertically, direct the telescope at a low-lying star.

Observe the reflected image of the star at the very edge of the mirror of the horizon glass, that is, the dividing line between the silvered and unsilvered part of the horizon glass. Turn the micrometer drum slowly until the true and reflected stars are exactly in coincidence and read the vernier or micrometer head.

If the reading is exactly zero there is no I.E. Should the reading be on the arc e.g. 2’ 40” then this would be an I.E. of 2’40”, to be subtracted from all measurements made with that sextant? I.E. is usually abbreviated to ‘IE’ and this would be written as IE – 2′ 40″. However, should the reading be 2′ 40″ off the arc, then this would be an I.E. of 2′ 40″ to be added to all observed angles, written as IE + 2′ 40″.

By daylight, the most common method of finding the I.E. is by observing the sea horizon as described above for the Third Adjustment.

With the index set approximately at zero and the sextant held vertically, point the telescope directly at the sea horizon and turn the micrometer drum until the true and reflected horizons appear to form one continuous line as in fig. 12-5 (b). Read the sextant and ascertain the I.E. as described in the last paragraph.

Another method of finding the I.E. in daylight is with the Sun when this is available. The index is set to about 0° 32.0 (the approximate diameter of the Sun) on the arc, and with the necessary shades in position, and the sextant held vertically, the Sun is observed through the telescope. The reflected image of the Sun will appear below the true image and by careful adjustment of the micrometer or tangent screw the two limbs (edges) of the Sun can be made to just touch each other as shown in fig. 12-5 (a).      

Read the angle of the sextant and note it down. Next, set the index to about 0° 32.0 off the arc and again observe the Sun, where, on this occasion, the reflected image will appear above the true image. Again bring the two limbs into exact contact as in fig. 12-6 (b) and read the sextant.  The I.E. is equal to half the difference between the two readings and is named ‘on’ or ‘off’ the arc according to whichever reading is the greater.

When the Sun is at low altitude the vertical diameter may be distorted by refraction and on such occasions, it is best to observe the horizontal diameter. 

This is done in exactly the same way as described above for observing the vertical diameter, except that this time the sextant is held horizontally. The left limb of the reflected image of the Sun is brought into contact with the right limb of the true image at about 32′ on the arc [fig. 12-6 (a)], then the right limb of the reflected image brought into contact with the left limb of the true image at about 32′ off the arc.             

Several sights (say three) should be taken on the arc and the same number off the arc, and a mean of the readings used to determine the I.E. It is difficult to get these angles correct so the limbs of the Sun should be placed alternately a little open and a little overlapping so that in making the contact the micrometer drum or tangent screw may be turned different ways. As a check on the accuracy of the observations, the sum of the two readings divided by four is the Sun’s Semi-Diameter for the day, which may be compared with that given In the Admiralty N.A. or equivalent commercial almanac.

Using the readings found in fig. 12-6 above, the I.E. would by found as follows: –

49. THE FOURTH ADJUSTMENT – COLLIMATION ERROR

If the optical axis of the sextant telescope is not parallel to the plane of the instrument, then collimation error exists. With modern, well-made sextants it is almost impossible for the telescope to get out of line unless the instrument has been knocked or the rising-piece bent, which would be obvious to a careful observer.

Before checking for collimation error, perpendicularity error and side error must be removed. Then ship the inverting telescope with the wires parallel to the plane of the instrument. Choose two celestial bodies not less than 90′ apart and bring them into accurate contact on one wire of the telescope. Move the sextant until the bodies are on the other wire, when the two bodies should still be in contact.  If they are not, then  collimation error exists.

Adjustment for collimation error is made by the two opposing screws: (A-A in fig. 12-7) in the double telescope collar. (It should be noted that some sextants have no screws provided to adjust for collimation error.) If the two images of the celestial bodies appear to separate when transferred from one wire to the other, the screw farthest from the frame of the sextant (the top screw) should be slackened and the other screw (the bottom one) tightened. On the other hand, should the images overlap, this procedure should be reversed (see fig. 12-7).

50. NON-ADJUSTABLE SEXTANT ERRORS

Most modern sextants are manufactured by highly skilled craftsmen who have at their disposal tools of the highest precision. Biodern sextantel, therefore, leave the bands of the makers in a state, as near to perfection as in possible, however, it is not unlikely that many old sextants do possess certain errors, which are not capable of being adjusted. The non-adjustable sextant errors are: –

Centring Error – due to the axis of the index bar not being in the centre of the circle of which the arc forms a part. It is very likely that an old well-used sextant possesses centring error on account of the wear of the bush on which the index bar pivots the resultant error varies in amount and sign over different parts.

Shade Error – due to the two surfaces of a Shade glass not being ground exactly parallel to each other. This cannot be adjusted but if suspected may be ascertained by taking an observation with the particular Shade. Noting the reading and then taking the same observation without the Shade. Any difference between the two readings will be Shade error, and should be carefully recorded.

Graduation Error – due to inaccuracies in the graduation of the arc, the vernier or the micrometer.

Worm and Rack Errors – Peculiar only to micrometer sextants. Until a sextant has been “run in” it is liable to small and progressive changes and for this reason, it is advisable to have a micrometer sextant recalibrated periodically according to the amount of use it has received.

Generally speaking, it may be said that the whole of the non-adjustable errors may be ignored in any modern sextant of good make, particularly if it has an N.P.L. Certificate, or to a lesser extent, a Maker’s Certificate.

If an observer, however constantly obtains different readings from a second observer after all the adjustments have been made and the known errors applied, he should have the sextant overhauled by the makers or a reliable Nautical Optician. In the event of any replacement to the original glassware, the sextant should always be re-calibrated.

51. USE AND CARE OF A SEXTANT

• When holding a sextant many navigators prefer to grasp the frame of the sextant around the handle as well, thus obtaining a firmer grip.
• The Instrument should never be left exposed to the Sun for longer than necessary between observations. It is strongly advisable to stow it away in its box to prevent damage, stowed away from damp and vibration.
•  When releasing the index bar clamp, firm pressure should be applied between thumb and finger while the bar is moved to prevent damage to the worm and rack. On re-securing the clamp, ensure the worm engages firmly with the rack on the underside of the arc by turning the micrometer drum or tangent screw a half turn either way.
• The arc and micrometer teeth must be kept clean in order to prevent any stiffness occurring in the movement of the index bar.
• At frequent intervals, the working parts of the sextant should be lightly smeared with the special oil provided to reduce the rate of wear.
• Never polish the arc with any abrasive substance or metal polish. It may sometimes be necessary to rub the are with a little lamp h. black and sweet oil and then wipe it off; this cleans the arc and makes it much more legible. An occasional rub of the arc with the bare finger will enable the reading to be made more easily. Be careful not to use anything that will cut the arc.
• After using the sextant, particulars In wet or rough weather, the surfaces of the index mirrors horizon glass and telescope lenses should be wiped carefully and lightly with a piece of soft chamois leather which is kept in the sextant box especially for the job. All moisture on these glass surfaces should be removed, particularly down the dividing line between the plain glass and silvered glass of the Horizon Glass; otherwise, the silvering will soon deteriorate and make observations difficult. Care must be exercised not to apply excessive pressure on the mirrors or the adjustments may be unset.
• To keep mirrors clean and bright use alcohol, fresh water, and tissue paper.
• Never torment or over-adjust a sextant. A modern sextant should remain in perfect adjustment for an indefinite period providing it is not knocked or subject to excessive vibration.
• When adjusting is necessary, it is essential to perform the First Adjustment (for Perpendicularity) before adjusting for Side Error and I.E. simultaneously (as described earlier).
• In small vessel’s particularly in lively weather, a neck lanyard should be attached to the sextant to prevent it’s falling overboard. The sextant should always be steadied with one hand when the observer is moving to prevent the lanyard acting as a pendulum and swinging the instrument against fittings.