The four errors in a sextant
1. Collimation error: This results when the telescope bracket is bent. It cannot be fixed without precision machine tools. Correct for this by removing the telescope.
2. Perpendicularity of the index mirror error: Adjust when sighting from the top of the sextant down past the index mirror to the arc, lining up the arc with the reflected image in the mirror. This is corrected using the screw on the side of the index mirror.
3. Side error: This is caused if the horizon glass is not parallel to the index mirror. Set the sextant to 0° 0’. Use a vertical object (or hold the sextant sideways to use the horizon) and make sure that there is only one clear image. If there are two images, the sextant has side error. This is corrected using the screw on the side of the horizon mirror.
4. Index error: Set the sextant to 0° 0’ and look out at the horizon through the eyepiece. There should be two horizons (if not, twirl the micrometer drum until the horizon splits in two). Match the two horizons and read the micrometer drum to determine the error. If it is necessary to adjust, use the screw on the top of the horizon mirror.