ATP-3-09-02 Field Artillery Survey Download
Page 92 of 168
Chapter 7 7-4 ATP 3-09.02 16 February 2016 adequate for timekeeping, if a correction is carried. For astronomic observation, a good watch is one that gains or loses a constant amount of time over a given period. The timekeeper should not try to set his watch to the exact time, but he must ensure that the second hand is in the vicinity of 12 when the minute hand is on a minute mark. (This will preclude a 30-second error.) The timekeeper will determine the amount his watch is in error and note the correction, with the proper +/-, in the remarks section of the recorded field notes. Sighting Techniques (Obtaining A Correct Sight-Picture) CAUTION The sun must never be viewed through the telescope without a sun filter. The filter should be inspected before use to ensure that the coated surface is free from scratches or other defects. Serious eye damage will result if proper precautions are not taken. If the sun filter has been damaged or lost, a solar observation may be completed by use of the card method. The image of the sun is projected onto a card held 3 to 6 inches behind the eyepiece and the telescope is focused so that the cross hairs are clearly defined. 7-15. When the sun is being observed, special sighting techniques are required to resolve its center because of the size and brilliance of the sun. Since the angular diameter of the sun is about 9.5 mils of arc, an accurate sight-picture cannot be achieved without the use of special aids and/or techniques. 7-16. One of the special sighting aids is the solar circle etched on the reticle of the observing instrument. Most theodolites have the solar circle etched on the reticle (see Figure 5-2). No special sighting technique is required with these instruments, but the sun must be centered in the circle. The sun will not always fit exactly into the circle. However, the amount of overlap, or spacing, will not affect the final result. 7-17. A correct sight-picture on a star is made in the same manner as sighting in on the sun, except that at the instant TIP is announced the cross hairs should bisect the star. Note: When using the Artillery Astronomic Method of observation, the instrument operator takes three direct readings to the celestial body, and then plunges the telescope, and sights back on the azimuth mark. A second set should be taken with three readings in the reverse position as a check on the instrument and operator. As a rejection criterion, the closing readings on the azimuth mark should agree with the initial circle setting by the known spread of the instrument. In addition, the mean of the two sets of readings should agree with both sets within prescribed accuracies. Note: When the observer is observing the stars it is advantageous for him to have the telescope blacked out until the star is identified. When the star has been identified, the telescope light rheostat is turned up so that as many stars as possible, other than the desired one, will be obliterated by the light in the telescope. Recording and Meaning Data 7-18. The format for recording field data and determining the mean angles is generally the same as that for other angle measurements. Sun observations are recorded in the same manner as star observations, Figures 4-5 through 4-8, are examples of how data is recorded in a field notebook for astronomic observations. Artillery Astronomic Using the Sun 7-19. In the artillery astronomic method of determining azimuth; two sides of the Pole-Zenith Star triangle, the polar distance and colatitude, and one angle are used to solve for the azimuth angle. This computation is