Sextant lrg

Grunt Productions 2007

Marine SextantMarine SextantReference:Reference:

Any Navigational text Book.Any Navigational text Book.

A brief by Lance Grindley.A brief by Lance Grindley.


Covering:Covering:

Basic construction of the Basic construction of the sextant.sextant.

Correctable sextant errors.Correctable sextant errors. Care of a sextant.Care of a sextant. Notes on taking sights.Notes on taking sights.


Marine SextantMarine Sextant So called because the length of the graduated So called because the length of the graduated

arc is one-sixth of a circlearc is one-sixth of a circle The angle of reflection is twice the angle of The angle of reflection is twice the angle of

incidence so sextant reads up to 120incidence so sextant reads up to 120oo

Parts of the sextant:Parts of the sextant:– Main frameMain frame with the graduated arc at the with the graduated arc at the

edgeedge– Index barIndex bar which rotates the index (top) which rotates the index (top)

mirrormirror– Index glassIndex glass which rotates at the arc’s which rotates at the arc’s

centrecentre– Horizon glassHorizon glass which is part silvered/part which is part silvered/part

clearclear


Marine SextantMarine Sextant


Sextant ErrorsSextant Errors

PerpendicularityPerpendicularity



Cause Cause - index glass not - index glass not perpendicular to the plane of the perpendicular to the plane of the instrumentinstrument

TestTest - put index arm at - put index arm at approximately 25approximately 25oo and check the and check the direct and reflected image of the arcdirect and reflected image of the arc

AdjustmentAdjustment - adjust the screw on the - adjust the screw on the index mirror until both images of the index mirror until both images of the arc are lined uparc are lined up



Arc of the Sextant

Reflected Image



PerpendicularityPerpendicularity Side ErrorSide Error


Side ErrorSide Error

CauseCause - horizon glass not - horizon glass not perpendicular to the plane of the perpendicular to the plane of the instrumentinstrument

TestTest - Direct and reflected image of - Direct and reflected image of the same star should appear as one the same star should appear as one when the arc is moved through zerowhen the arc is moved through zero

AdjustmentAdjustment - adjust the outer screw - adjust the outer screw on the mirror on the mirror


Side ErrorSide Error

No Side Error Side Error Present



PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error


Collimation ErrorCollimation Error

CauseCause - telescope is not parallel to - telescope is not parallel to the plane of the instrumentthe plane of the instrument

TestTest - Use inverting telescope with - Use inverting telescope with cross-wires parallel to the plane of cross-wires parallel to the plane of the instrument and view two objects the instrument and view two objects at least 90at least 90oo apart. apart.

AdjustmentAdjustment - collar on the telescope - collar on the telescope but this should not be loosenedbut this should not be loosened


Collimation ErrorCollimation Error

No CollimationError

CollimationPresent



PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error Index ErrorIndex Error


Index ErrorIndex Error

Caused by mirrors not being parallel Caused 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 Observe a star (make images

coincident)coincident)– Observe the horizon (Make images Observe the horizon (Make images

continuous)continuous)



Index Error “ON” the Arc

TRUESUN

Reading



Index Error “OFF” the Arc

TRUESUN

Reading



Index Error is:Index Error is:– Difference between readings “on” and Difference between readings “on” and

“off” the arc divided by 2“off” the arc divided by 2 Index error is in the direction of the Index error is in the direction of the

largest reading - added if “off” the largest reading - added if “off” the arc and subtracted if “on” the arcarc and subtracted if “on” the arc

Addition of reading “on” and “off” Addition of reading “on” and “off” the arc should equal 4 times SD for the arc should equal 4 times SD for the day the day



PerpendicularityPerpendicularity Side ErrorSide Error Collimation ErrorCollimation Error Index ErrorIndex Error Non-adjustable errorsNon-adjustable errors

– Centering ErrorCentering Error– BacklashBacklash– Optical ErrorsOptical Errors


Sextant CalibrationSextant Calibration

If badly knocked, dropped or If badly knocked, dropped or physically damagedphysically damagedThe mirrors lose their reflective The mirrors lose their reflective coatingcoatingParts of the equipment are missingParts of the equipment are missingThere is strong reason to suspect There is strong reason to suspect the accuracy of the instrumentthe accuracy of the instrumentThe calibration certificate is more The calibration certificate is more than 10 years oldthan 10 years old


Care of Sextant:Care of Sextant:Take care when screwing telescope into Take care when screwing telescope into collarcollarAlways use safety lanyardAlways use safety lanyardNever leave lying exposed to the SunNever leave lying exposed to the SunWipe spay or moisture off mirrors Wipe spay or moisture off mirrors immediately after useimmediately after useUse oil/Vaseline to preserve the arcUse oil/Vaseline to preserve the arcReplace carefully in box with shades Replace carefully in box with shades closed and index bar set for packing closed and index bar set for packing piecespieces


Notes on Sextant:Notes on Sextant:

Test for adjustable errors before useTest for adjustable errors before useEnsure mirrors are fixed securelyEnsure mirrors are fixed securelyFocus telescope on infinity (horizon)Focus telescope on infinity (horizon)Take observations in the centre of the Take observations in the centre of the telescope’s field of viewtelescope’s field of viewObserve very small angles on/off the Observe very small angles on/off the arcarcUse telescope shade in preferenceUse telescope shade in preferenceAlways have cloth/paper to wipe glassesAlways have cloth/paper to wipe glasses


Taking Sights:Taking Sights:Calculate expected altitude/bearing firstCalculate expected altitude/bearing firstTake observations with best horizonTake observations with best horizonAlways swing sextant for good altitudeAlways swing sextant for good altitudeIn poor visibility reduce height of eyeIn poor visibility reduce height of eyeUse strong shades when observing the Use strong shades when observing the SunSunTake multiple observations of each Take multiple observations of each body in poor conditionsbody in poor conditionsPossible to take observations in strong Possible to take observations in strong moonlight but treat with caremoonlight but treat with care


Questions??Questions??

Sextant lrg

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