DAVIS Mark 15, 25 Sextant - Manual (Page 6)

Page 4
Adjusting for and Calculating Built-In Index Error
Adjusting your sextant is easy and should be done each time it is used. On a cor-
rectly adjusted sextant, the two mirrors are always perpendicular to the frame and
become parallel to each other when the body and drum scales read zero.
To initially adjust the index mirror so that it is perpendicular to the
frame:
1. Set the instrument at approximately 50°.
2. Holding the sextant horizontal and about eight inches from the eye, look
with one eye into the mirror so that the frame arc is reflected in the mir-
ror.
3. Move the instrument until you can look past the index mirror and see the
actual frame arc as well as the reflected arc.
The two arcs should appear as one continuous curve (Fig. 3). If they do not,
turn the adjustment screw at the back of the index mirror until the two arcs
come into alignment.
To adjust the sextant for index error:
1. Set the instrument at 0° 00' and look at the horizon.
2. Keeping the sextant close to your eye, turn the screw that is furthest
from the frame at the back of the horizon mirror until the two horizon
images move exactly together (Fig. 4).
The two mirror are now parallel.
Figure 3
Figure 4