ABSTRACT
Instrumentation employed in the marine industry continues to go through radical change on the back of the microchip. In marine sextant, there are three main errors which can quite easily be corrected by the mariner. A fourth error, for 'collimation', can also be corrected, with care and attention, but only to an older sextant where telescope collars are fitted with adjusting screws. Collimation error is caused by the axis of the telescope not being parallel to the plane of the instrument. This error can be corrected by adjustment of the two screws in the collar or telescope mounting. Perpendicularity is caused by the index mirror not being perpendicular to the plane of the instrument. Side error is caused by the horizon mirror not being perpendicular to the plane of the instrument. Index error is caused by the index mirror and the horizon mirror not being parallel to each other when the index arm is set at zero.
