Sextant

Question 1

What is the Sextant?

Answer 1

• The sextant is a precision instrument used to measure both the vertical and horizontal angles.

Question 2

What is the 1st correctable error?

Answer 2

Error of Perpendicularity?
 This is when the Index Mirror is not perpendicular to the plane of the sextant.
 How to check:
 Set the index arm between 30o- 40o on the arc.
 Hold the sextant horizontally outwards.
 Check if the true image and reflected image are the same.

Question 3

How to correct 1st error?

Answer 3

 Use the adjustment screw behind the Index Mirror until the reflected image is the same as the true.

Question 4

What is the 2nd correctable error?

Answer 4

(Side Error)
 This is when the horizon mirror is not perpendicular to the plane of the instrument.
 How to check by horizon:
 Set the index arm to 0o on the arc.
 Hold the sextant just out of the horizontal position.
 Check if the two images are misaligned.
 How to check by a star:
 Set the index arm to 0o on the arc.
 Hold the sextant vertically and select a star.
 Observe if the stars are side by side, if so then-No Error is found, and no correction is needed.

Question 5

How to correct 2nd error?

Answer 5

 Use the adjustment screw behind the Index Mirror that is furthest from the plane of the instrument until the two images are side by side.

Question 6

What is the 3rd correctable error?

Answer 6

(Index Error)
 This is when the Index Mirror and the Horizon Mirror are not parallel to each other when the Index arm is set to 0o.
 How to check:
 Set the Index arm to 0o.
 Hold the sextant vertically.
 Look through the telescope and check if the two horizons are misaligned.

Question 7

How to correct 3rd error?

Answer 7

 Use the adjustment screw on the back of the horizon mirror closest to the plane of the instrument.

Question 8

What are the sextant non-correctable errors?

Answer 8

• Centring error:
   This occurs when the Index arm does not pivot at the exact centre of the arc.
• Prismatic error:
   This is caused when two faces of the mirrors are either, warped, cracked or not parallel.
• Shade error:
   This is caused when the faces of the shade are either, warped, cracked or not parallel.
• Graduation error:
   This happens when the arc, vernier or micrometre scales are improperly cut or incorrectly calibrated.
• Collimation error:
   This occurs when the telescope is not parallel with the plane of the instrument.
• Worm and Rack:
   This is found when there is a fault in the work and rack gearing between the micrometre and the main arc.