How to Check and Correct Sextant Errors ?

How to check perpendicularity error of sextant ?
  • Remove telescope
  • Turn away the shades
  • Set index arm about the middle of arc
  • look into mirror so the true arc is visible and reflected arc seen in mirror
  • if true and reflected arcs are not in a straight line error is present.
CORRECTION:-
  • To remove perpendicularity error makes adjustment the first adjustment until true and reflected arcs are appear in a straight line.
How to check  side error of sextant ?
  • Remove telescope
  • Turn away the shades
  • Set index arm and micrometer exactly at zero.
  • hold the sextant horizontally and look through the horizon mirror at horizon.
  • If true and reflected arcs are not in a straight line error is present.
  • Or hold sextant in vertical position and look directly 2nd magnitude star through telescope.
  • If true and reflected images pass to the side by side error is present.
CORRECTION:-
  • To remove error makes adjustment the second adjustment until true and reflected horizon or image of star are appear in a straight line.
How to check  index error of sextant?
  • Set index arm and micrometer exactly at zero.
  • hold sextant vertically and look through the telescope and horizon mirror at horizon or 2nd magnitude star. If true and reflected horizon is not in a straight line then there is index error .to find index error adjust micrometer until they appear in a straight line.

Or                                

  • Adjust micrometer at 32’ off the arc .
  • Hold sextant vertical position and look through the telescope at the sun. Move the micrometer until lower limb of reflected image just touches the upper limb of the true image.
  • Note reading and label it “off” the arc.
  • adjust micrometer at 32’ on the arc.
  • hold sextant vertical position and look through the telescope at the sun. Move the micrometer until upper limb of reflected image just touches the lower limb of the true image.
  • note reading and label it “on” the arc.
  • Index error = (( greater reading )- smaller reading (name index error same as greater reading) /2

Checking accuracy of an observation: Sun’s semi – diameter = (greater reading + smaller reading) /4

CORRECTION:-
  • To remove error makes adjustment the third adjustment until true and reflected horizon or image of star are appear in a straight line.
How to check collimation error  of sextant ?
  • Place sextant on level table
  • Look in line midway between the telescope and plane of instrument and check telescope is parallel to the plane of instrument.
CORRECTION:-
  • Observed low altitude star and cross wires set parallel to the plane of instrument
  • Observed another 2 celestial bodies 90 – 120 apart and true and reflected images are brought into coincidence on the left cross wires.
  • From different position bring the two bodies into coincidence on the right cross wire.
  • if images separate when transferred from left to right error is present.
  • by attaching telescope collar make telescope parallel to the plane of the instrument by adjustment screws also consult manual.
Why the index error of the sextant is substracted when it is “on the arc” and added when it is “off the arc”?

When we take altitude of any terrestrial, celestial bodies or horizontal angle of terrestrial bodies then angle start at point where true and reflected image appear in a straight line.

About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

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