Part I · Foundations · Page 4 of 5
Adjusting the sextant
A sextant has three adjustable errors. Check them in this order, because each adjustment can disturb the next:
- Perpendicularity: the index mirror must be square to the frame. Set the arm near 35°, look into the index mirror from behind it: the arc seen directly and its reflection should run on as one line.
- Side error: the horizon glass must be square to the frame. With the arm at zero, look at the horizon and tilt the sextant: if the two horizons step apart, adjust.
- Index error: what is left with the sextant upright. A small index error is simply measured and allowed for, as on the page about index correction; a large one is adjusted out.
- Sextant tilted
- 40°
- Step between the horizons
- 1.3′
A step that appears when you tilt the sextant is side error: the horizon glass is not square to the frame. Adjust it (the screw furthest from the frame) before the index error.
A fourth, collimation (the telescope not parallel to the frame), matters only for large angles and is set at the factory on most sextants.