Meade LX90GPS LX90 Tips The Global Positioning System, Detecting True Level, Finding True North

GPS

Receiver

Fig. 9: The GPS Receiver.

LX90 Tips

The Global Positioning System

The Global Positioning System (GPS) is comprised of 24 satellites orbiting the Earth that are constantly transmitting their precise position and time. The system provides highly accurate, worldwide positioning and navigation information for any number of applications.

GPS receivers on the Earth acquire signals from three to twelve satellites to determine the precise latitude, longitude, and time of the receiver. (Accuracy of the receiver's position may be within 10 to 15 feet.) As AutoStar uses latitude, longitude, and time information to calculate the positions of celestial objects, GPS is an ideal tool for aligning your your telescope.

Detecting True Level

To detect level of the base of the telescope, AutoStar must calculate the tip and tilt of the telescope at three compass points and then compensate for it. Finding level involves the geometric calculations of a "plane." In order to define a plane, three positions are necessary. This is not unlike building a table: For a table to stand level and solid, it must have a minimum of three legs. AutoStar makes gravitational measurements to make a precise determination of true level.

Finding True North

Locating True North is one of the most important ingredients in the alignment of a telescope. True North is the axis—the pole—which the Earth spins about and is a key reference for the motion of the Earth.

As you look at the night sky, the stars seem to move; in fact, if you watched long enough or took a long time-exposure photograph, you'd realize that the stars seem to revolve around one point—the pole or True North. When AutoStar knows where True North is and also knows the time, it can calcu- late the location of all the other objects in the sky.

One traditional way to find True North is to locate the North Star, Polaris, which lies very close to True North. Another way to calculate True North is to use gyroscopes or accelerometers.

LX90's determine True North by using a magnetic North sensor. The sensor locates magnetic North. Magnetic North is not True North, but a measure- ment of the magnetic lines of the Earth. Magnetic North may deviate several degrees from True North. But AutoStar, using the observation site location determined by the GPS and magnetic North information, can calculate the position of True North.

Some areas are subject to magnetic disturbances, and the magnetic field of the Earth changes slightly from year to year. AutoStar allows you to adjust for discrepancies in the local magnetic field using the "Calibrate Sensors" option in the Telescope menu. See page 29 for more information.

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