The sextant has an index error if the index mirror and horizon mirror are not parallel when the index arm (alidade) and the drum with minute scale are set exactly at zero. ... the “index error” must be removed or reduced using the adjusting screw on the horizon mirror (adjustig screw no. 2).

How do you fix an index error in a sextant?

To correct for index error, you first need to work out if it is present. Set the sextant to 0° and then focus on something in the distance. The horizon is ideal. If the horizon looks continuous, the sextant has no index error.

What is the index error?

the error in the reading of a mathematical instrument arising from the zero of the index not being in complete adjustment with that of the limb, or with its theoretically perfect position in the instrument; a correction to be applied to the instrument readings equal to the error of the zero adjustment. See also: Index.