Editing Sextant (section)

==Adjustment==
Due to the sensitivity of the instrument it is easy to knock the mirrors out of adjustment. For this reason a sextant should be checked frequently for errors and adjusted accordingly.

There are four errors that can be adjusted by the navigator, and they should be removed in the following order.

;Perpendicularity error:This is when the index mirror is not perpendicular to the frame of the sextant. To test for this, place the index arm at about 60° on the arc and hold the sextant horizontally with the arc away from you at arm's length and look into the index mirror. The arc of the sextant should appear to continue unbroken into the mirror. If there is an error, then the two views will appear to be broken. Adjust the mirror until the reflection and direct view of the arc appear to be continuous.
;Side error:This occurs when the horizon glass/mirror is not perpendicular to the plane of the instrument. To test for this, first zero the index arm then observe a star through the sextant. Then rotate the tangent screw back and forth so that the reflected image passes alternately above and below the direct view. If in changing from one position to another, the reflected image passes directly over the unreflected image, no side error exists. If it passes to one side, side error exists. Alternatively, the user can hold the sextant on its side and observe the horizon to check the sextant during the day. If there are two horizons there is side error. In both cases, adjust the horizon glass/mirror until respectively the star or the horizon dual images merge into one. Side error is generally inconsequential for observations and can be ignored or reduced to a level that is merely inconvenient.
;Collimation error:This is when the [[telescope]] or [[monocular]] is not [[Parallel (geometry)|parallel]] to the [[Plane (mathematics)|plane]] of the sextant. To check for this you need to observe two stars 90° or more apart. Bring the two stars into coincidence either to the left or the right of the field of view. Move the sextant slightly so that the stars move to the other side of the field of view. If they separate there is [[collimation]] error. As modern sextants rarely use adjustable telescopes, they do not need to be corrected for collimation error.
;Index error:This occurs when the index and horizon mirrors are not parallel to each other when the index arm is set to zero. To test for index error, zero the index arm and observe the horizon. If the reflected and direct image of the horizon are in line there is no index error. If one is above the other adjust the index mirror until the two horizons merge. Alternatively, the same procedure can be done at night using a star or the Moon instead of the horizon.