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Photo Gallery: Polar (Equatorial) Mounts

In the field of astronomy, a polar mount is frequently called an equatorial mount, and the hour angle axis is often called the polar axis. But by any combination of terms, the underlying principle is the same: in both satellite communications and astronomy, the hour angle axis must be parallel to the earth's polar axis (i.e., perpendicular to the earth's equatorial plane).

This adjustment is made by aligning the hour angle axis so that:
• The axis lies in a vertical plane oriented north-south.
• The angle between the hour angle axis and horizontal is equal to the local latitude angle.
This procedure is often referred to as "setting the latitude angle."

Once this adjustment has been made:
• A properly-aligned telescope (optical or radio) can be set to any desired declination. Once declination is set, any astronomical object can be tracked by adjusting only the hour angle.
• A properly-aligned satellite antenna must be set to the declination appropriate to its latitude.

Once declination set correctly, any satellite in the Clarke Belt can be reached by adjusting only the hour angle. Following are illustrations of polar/equatorial mounts:

Satellite Antenna on Fixed Polar Mount: 10-foot antenna on fixed polar mount.  The latitude angle Ø and the declination angle are fixed when the antenna is installed, and locked into position.  Rotation about the hour angle axis is adjusted by manually turning an adjustment bolt.

Satellite Antenna on Steerable Polar Mount: Comtech Antenna Systems, Inc. 5.0-meter antenna on motor-driven polar mount. Axis labels.   The latitude angle Ø and the declination angle are fixed when the antenna is installed, and locked into position.  Rotation about the hour angle axis is adjusted by means of a linear actuator driven by a motor.

Small Optical Telescope: Meade Model 4420 4.5-inch Newtonian Telescope on non-motorized "German" equatorial mount.  The hour angle axis is set to the correct latitude angle by adjusting the latitude angle Ø.  Rotation about the hour angle and declination axes is adjusted manually using adjustment knobs.

Small Optical Telescope: Celestron Model C90 4420 Schmidt-Cassegrain Telescope on motorized "wedge" equatorial mount.  The hour angle axis is set to the correct latitude angle by adjusting the latitude angle Ø.  Rotation about the hour angle and declination axes is set manually; once set, declination is locked into position and the motor continuously moves the hour angle to compensate for the earth's rotation.

Large Optical Telescope: 60-inch telescope at the Mount Wilson Observatory, near Pasadena, California, USA. The polar axis is fixed at an angle of approximately 34° with respect to the floor; this angle equals the latitude at Mount Wilson.

Large Radio Astronomy Antenna: 140-foot radio astronomy antenna at National Radio Astronomy Observatory, Green Bank, West Virginia, USA. The hour angle (polar) axis is fixed at an angle of approximately 38°; this angle equals the latitude at Green Bank.   Axis labels.

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