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worse collimation.
Figure 13.3. Correct and Incorrect Collimation.
AFPAM11-216 1 MARCH 2001 281
13.4.2. Bubble size affects the accuracy of a sextant observation. The ideal situation for collimation is to
have a small bubble for ease in determining the center. A bubble that is too small will stick to the lens,
decreasing accuracy. A bubble that is too large will move like a creature from a science fiction movie,
making it difficult to find the center. Experience shows that best results are obtained with a bubble
approximately one and a half times the apparent diameter of the sun or moon, or about the size of a
Cheerio. The field prism is geared to an altitude scale so that when the body is collimated the altitude
can be read from the scale.
13.4.3. An averaging mechanism is also incorporated which allows the navigator to take an observation
over a period of time. The continuous motion of the aircraft affects the bubble and resultant artificial
horizon. This movement resolves itself into a cycle in which the aircraft rolls, yaws, and pitches. To
obtain an accurate reading, it is necessary to sight the body for a period of time during this cyclic
movement and to average the results of a series of sightings. An averaging device has been incorporated
in the sextant so an average reading can be obtained.
13.4.4. The sextant (Figure 13.4) is actually a low-power periscope with a 15o field of view. All lens
surfaces in the sextant are coated to minimize light loss. To prevent condensation when the tip of the
sextant is extended into cold air, the tube is filled with a dry gas and sealed. A desiccant (composed of
silica gel) is used to remove moisture and check on the dryness of the gas inside the tube, and is visible
in the periscopic end of the sextant, or in some models, on the sextant body. When the silica gel is pink,
there is moisture in the tube and the sextant should be replaced before flight.
NOTE: The numbers in parentheses in 13.4.5 through 13.4.11 refer to the parts indicated in Figure 13.4.
Figure 13.4. Periscopic Sextant.
282 AFPAM11-216 1 MARCH 2001
13.4.5. An eyepiece (1) rotates to correct the eyesight of the individual observer. Filters (2) are provided
for selective use in the optical system so that the intensity of the sun's light might be adequately reduced.
The filter control (2) is located on the left of the sextant.
13.4.6. Most sextants currently in use have been modified with an electronic device for accomplishing
all the functions of the averaging mechanism. General differences in these and the unmodified sextants
are addressed in this discussion.
13.4.7. A start switch (4) (a start and stop or averager operating lever [4A] on unmodified sextants)
starts and stops the operation of the sextant. Adjacent to this switch is the reset switch (5) (the averager
rewind lever, if unmodified [5A] located below the averager operating lever). The reset switch or
averager rewind lever has four functions. When depressed and released, it does the following: (1)
removes the shutter from the field of vision, (2) zeroes and resets (rewinds if unmodified) the timer, (3)
zeroes the averager and places initial values in registers and data memory (realigns indices on
unmodified sextants), and (4) disconnects the altitude control knob from the averager.
13.4.8. The bubble control knob (6) should be left in the maximum increase position after adjustments
have been made. With the control in the maximum increase position, an aneroid is locked to the bubble
chamber to compensate for changes in ambient pressure and temperature.
13.4.9. On the front of the sextant, there is a rheostat control (7) which varies the intensity of the light in
the bubble chamber. The altitude knob (8) is located on the right side of the sextant. It keeps the
observed body in vertical collimation during the period of the observation. At the end of the scheduled
observation, it adjusts the altitude counter until the exact average indication appears, or to align the
indices on unmodified sextants. The body's altitude is read in the altitude counter (9). Directly behind
the altitude knob is the averager display (10) (half-time dial and indices if unmodified). The averager
display or half-time dial is graduated from 0-60 and indicates the half time of the observation. The
indices, when aligned, permit the direct reading of the observed altitude on the altitude dial.
13.4.10. In the periscope sextant, the averaging is accomplished by microprocessor (Deimel-Black ball
integrator if unmodified), which effects a continuous moving averager over any observation period up to
2 minutes. This system has many advantages over other known averaging devices: it is very simple to
operate. A single switch (or lever) sets or winds the mechanism and no other presetting of the sextant,
　
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