Sextant lrg
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The document summarizes the basic construction and use of a marine sextant. It describes the main components of a sextant including the frame, index bar, index glass, and horizon glass. It discusses correctable errors like perpendicularity, side error, and collimation error. It provides instructions on testing for errors and making adjustments. The document also covers care, calibration needs, and best practices for taking accurate sightings with a sextant.
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Sextant lrg
- 1. Grunt Productions 2007 Marine SextantMarine Sextant Reference:Reference: Any Navigational text Book.Any Navigational text Book. A brief by Lance Grindley.A brief by Lance Grindley.
- 2. Grunt Productions 2007 Covering:Covering: Basic construction of the sextant.Basic construction of the sextant. Correctable sextant errors.Correctable sextant errors. Care of a sextant.Care of a sextant. Notes on taking sights.Notes on taking sights.
- 3. Grunt Productions 2007 Marine SextantMarine Sextant So called because the length of the graduated arc isSo called because the length of the graduated arc is one-sixth of a circleone-sixth of a circle The angle of reflection is twice the angle of incidenceThe angle of reflection is twice the angle of incidence so sextant reads up to 120so sextant reads up to 120oo Parts of the sextant:Parts of the sextant: – Main frameMain frame with the graduated arc at the edgewith the graduated arc at the edge – Index barIndex bar which rotates the index (top) mirrorwhich rotates the index (top) mirror – Index glassIndex glass which rotates at the arc’s centrewhich rotates at the arc’s centre – Horizon glassHorizon glass which is part silvered/part clearwhich is part silvered/part clear
- 4. Grunt Productions 2007 Marine SextantMarine Sextant
- 5. Grunt Productions 2007 Sextant ErrorsSextant Errors PerpendicularityPerpendicularity
- 6. Grunt Productions 2007 PerpendicularityPerpendicularity CauseCause - index glass not perpendicular- index glass not perpendicular to the plane of the instrumentto the plane of the instrument TestTest - put index arm at approximately- put index arm at approximately 2525oo and check the direct and reflectedand check the direct and reflected image of the arcimage of the arc AdjustmentAdjustment - adjust the screw on the- adjust the screw on the index mirror until both images of the arcindex mirror until both images of the arc are lined upare lined up
- 7. Grunt Productions 2007 PerpendicularityPerpendicularity Arc of the Sextant Reflected Image
- 8. Grunt Productions 2007 PerpendicularityPerpendicularity Arc of the Sextant Reflected Image
- 9. Grunt Productions 2007 Sextant ErrorsSextant Errors PerpendicularityPerpendicularity Side ErrorSide Error
- 10. Grunt Productions 2007 Side ErrorSide Error CauseCause - horizon glass not perpendicular to- horizon glass not perpendicular to the plane of the instrumentthe plane of the instrument TestTest - Direct and reflected image of the- Direct and reflected image of the same star should appear as one when thesame star should appear as one when the arc is moved through zeroarc is moved through zero AdjustmentAdjustment - adjust the outer screw on the- adjust the outer screw on the mirrormirror
- 11. Grunt Productions 2007 Side ErrorSide Error No Side Error Side Error Present
- 12. Grunt Productions 2007 Side ErrorSide Error No Side Error Side Error Present
- 13. Grunt Productions 2007 Side ErrorSide Error No Side Error Side Error Present
- 14. Grunt Productions 2007 Sextant ErrorsSextant Errors PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error
- 15. Grunt Productions 2007 Collimation ErrorCollimation Error CauseCause - telescope is not parallel to the- telescope is not parallel to the plane of the instrumentplane of the instrument TestTest - Use inverting telescope with cross-- Use inverting telescope with cross- wires parallel to the plane of thewires parallel to the plane of the instrument and view two objects at leastinstrument and view two objects at least 9090oo apart.apart. AdjustmentAdjustment - collar on the telescope but- collar on the telescope but this should not be loosenedthis should not be loosened
- 16. Grunt Productions 2007 Collimation ErrorCollimation Error No Collimation Error Collimation Present
- 17. Grunt Productions 2007 Sextant ErrorsSextant Errors PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error Index ErrorIndex Error
- 18. Grunt Productions 2007 Index ErrorIndex Error Caused by mirrors not being parallelCaused by mirrors not being parallel when sextant is set at zerowhen sextant is set at zero Usually removed mathematicallyUsually removed mathematically Methods of checking:Methods of checking: – Observe Sun “on” and “off” the arcObserve Sun “on” and “off” the arc – Observe a star (make images coincident)Observe a star (make images coincident) – Observe the horizon (Make imagesObserve the horizon (Make images continuous)continuous)
- 19. Grunt Productions 2007 Index ErrorIndex Error Index Error “ON” the Arc TRUE SUN Reading
- 20. Grunt Productions 2007 Index ErrorIndex Error Index Error “OFF” the Arc TRUE SUN Reading
- 21. Grunt Productions 2007 Index ErrorIndex Error Index Error is:Index Error is: – Difference between readings “on” and “off”Difference between readings “on” and “off” the arc divided by 2the arc divided by 2 Index error is in the direction of theIndex error is in the direction of the largest reading - added if “off” the arclargest reading - added if “off” the arc and subtracted if “on” the arcand subtracted if “on” the arc Addition of reading “on” and “off” the arcAddition of reading “on” and “off” the arc should equal 4 times SD for the dayshould equal 4 times SD for the day
- 22. Grunt Productions 2007 Sextant ErrorsSextant Errors PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error Index ErrorIndex Error Non-adjustable errorsNon-adjustable errors – Centering ErrorCentering Error – BacklashBacklash – Optical ErrorsOptical Errors
- 23. Grunt Productions 2007 Sextant CalibrationSextant Calibration If badly knocked, dropped or physicallyIf badly knocked, dropped or physically damageddamaged The mirrors lose their reflective coatingThe mirrors lose their reflective coating Parts of the equipment are missingParts of the equipment are missing There is strong reason to suspect theThere is strong reason to suspect the accuracy of the instrumentaccuracy of the instrument The calibration certificate is more thanThe calibration certificate is more than 10 years old10 years old
- 24. Grunt Productions 2007 Care of Sextant:Care of Sextant: Take care when screwing telescope intoTake care when screwing telescope into collarcollar Always use safety lanyardAlways use safety lanyard Never leave lying exposed to the SunNever leave lying exposed to the Sun Wipe spay or moisture off mirrorsWipe spay or moisture off mirrors immediately after useimmediately after use Use oil/Vaseline to preserve the arcUse oil/Vaseline to preserve the arc Replace carefully in box with shadesReplace carefully in box with shades closed and index bar set for packingclosed and index bar set for packing piecespieces
- 25. Grunt Productions 2007 Notes on Sextant:Notes on Sextant: Test for adjustable errors before useTest for adjustable errors before use Ensure mirrors are fixed securelyEnsure mirrors are fixed securely Focus telescope on infinity (horizon)Focus telescope on infinity (horizon) Take observations in the centre of theTake observations in the centre of the telescope’s field of viewtelescope’s field of view Observe very small angles on/off the arcObserve very small angles on/off the arc Use telescope shade in preferenceUse telescope shade in preference Always have cloth/paper to wipe glassesAlways have cloth/paper to wipe glasses
- 26. Grunt Productions 2007 Taking Sights:Taking Sights: Calculate expected altitude/bearing firstCalculate expected altitude/bearing first Take observations with best horizonTake observations with best horizon Always swing sextant for good altitudeAlways swing sextant for good altitude In poor visibility reduce height of eyeIn poor visibility reduce height of eye Use strong shades when observing theUse strong shades when observing the SunSun Take multiple observations of each bodyTake multiple observations of each body in poor conditionsin poor conditions Possible to take observations in strongPossible to take observations in strong moonlight but treat with caremoonlight but treat with care
Editor's Notes
- A sextant, by the very nature of its task must be a precision instrument. An error of one minute of arc represents an error of one nautical mile in the final position line. To obtain the best results, the sextant errors need to be reduced to a minimum and care must be taken to reduce further errors in use. Sextant errors are easy to detect and correct and by following a few sensible rules, the effect of other sources of inaccuracy can be minimized. Current sextants cost about £1,200 to refurbish, so they need to be treated with care.
- Easily correctable sextant errors are perpendicularity and side error. Perpendicularity occurs if the index glass is not exactly perpendicular to the plane of the instrument, the graduated sextant arc. With the telescope removed, the error is found by setting the altitude at a low value whilst looking at the arc immediately to the right of the mirror and noting the position of the arc reflected in the mirror (the other end of the arc). The two images should appear to lie on a continuous arc. If perpendicularity is present, there will be a step between the two images. The index glass is easily adjusted using the screw at the back of the mirror. A device looking like a hat pin is provided in the box to make this adjustment. The check and correction for perpendicularity must be carried out first.
- Side error is caused by the plane of the horizon glass not being perpendicular to the plane of the instrument. To check for side error, the telescope is fitted and the sextant is set close to zero. Ideally a star should be observed but a vertical object such as a mast can be used. At zero altitude, the two images of the star should be coincident. If the two images appear side by side, side error is present. Adjustment is made in the same way as for perpendicularity. The outer screw is used to adjust the glass until the images are coincident. The inner screw is used to adjust index error and in most circumstances this should not be touched.
- Collimation error is listed in some manuals but it is both difficult to detect and impossible to adjust. It will only be present if the screw used for mounting the telescope has been damaged. Do not attempt to correct for this.
- Index error is usually the largest cause of error, and although it can be largely removed, it is usually corrected mathematically. Index error is caused because the planes of the index and horizon glasses are not exactly parallel when the sextant is set at zero. The mechanical correction is made using the inner screw on the horizon glass. Adjustment of this, however, would have some effect on side error. Correcting side error would than have a small effect on index error, and so on. Index error can be detected by viewing the horizon, a star or measuring the sun’s diameter “on” and “off” the arc. Measuring the diameter of the sun “on” and “off” the arc is the favored method because the result can be subsequently checked by comparing the result with the tabulated semi-diameter of the sun. IE is half the mean of the readings “on” and “off” the arc. The two readings combined should be equal to 4 times the tabulated SD. (The micrometer reading “off” the arc must be adjusted)
- A sextant may have non-adjustable errors. Centring error occurs when the pivot at the centre of the arc is not exactly at the centre of the graduated brass arc of the instrument. The error is detected during calibration and the results are shown on the graph fixed to the inside of the box lid. Backlash occurs when the reading obtained when moving the index bar in one direction is different to the reading when it is brought up in another direction. Optical errors are minimized and contribute to the overall cost of the instrument. Although the shades are optically ground, it is better to use the eyepiece shade in preference to the hinged shades. Sextant refurbishment and calibration is necessary when the instrument is returned to stores. As this is a costly process, the sextant should not be returned unnecessarily.
- The sextant can be damaged if the telescope collar is too close to the frame. To guard against damage and allow both hands to be free when required, a lanyard should be attached. If exposed to direct sunlight, the heating may not be uniform and the frame may be distorted. The sextant will inevitably get covered in spray, if not soaked from time to time. It must be cleaned and dried before putting away. It can be washed in fresh water. The arc must not be cleaned with “Brasso” as this removes the graduated scale, but a wipe with oil will keep the steel section inlay from getting rusty. The instrument should always be kept in the box when not in use.
- Check the sextant when removing it from the box. Check perpendicularity, ship the telescope and check side error if possible. Focus on the horizon and check IE. Observations taken in the centre of the field of view are less likely to be subject to optical error. The best sights are usually taken from the clearest horizon. Choose sights carefully and set the altitude to that expected for the selected star/planet. Check the bearing and then look carefully in that direction, using the sextant. Swinging the sextant ensures that the altitude is taken perpendicular to the horizon. Looking east, place the star just below the horizon and swing the sextant as it rises until it just touches. Record the time. Reverse this for the western horizon. Changing height of eye reduces the distance of the visible horizon. A change may give better results in rough weather or poor visibility. Most common error with sun sights is not using a dark enough shade, hence the limb is not seen clearly. Multiple observations may help to improve accuracy.