Polar aligning an equatorial mount

Establishing a solid base

Setting up an equatorial mount begins long before the stars appear. You must first find a flat, firm patch of ground where the tripod legs can rest without sinking into the soil. Use a small bubble level to ensure the tripod head is completely horizontal, adjusting each leg individually. Once the base is level, orient the tripod so that the mount’s polar axis — the tilted central axis of the assembly — points roughly toward geographic north. If you are waiting for twilight to deepen, you can pass the time with a pair of 10×50 binoculars to sweep the western sky. Ensuring this physical foundation is correct saves immense frustration later when making micro-adjustments in the dark.

Finding the celestial pole

To align the mount, you must aim its polar axis directly at the rotational pivot of the night sky. In the Northern Hemisphere, we lucky observers have a convenient guide star, Polaris, which sits just under three-quarters of a degree away from the true northern celestial pole. You will need to peer through the small, hollow polar scope built into the center of the mount’s right ascension axis. Looking through this tiny glass element for the first time can be disorienting. We look through the small internal optical tube and see… nothing but a faint gray field until our eyes adjust to the darkness.

Adjust the altitude screw on the mount until the scale matches your local latitude. This gets the target into the field of view of your polar scope. One veteran observer notes, “aiming the mount is a game of millimeters,” so make your adjustments slow and deliberate. You will see a small reticle pattern engraved on the glass inside the polar scope, often featuring a small circle where Polaris should be placed depending on the current date and time. Use the fine altitude and azimuth knobs on the mount — not the main clutch locks — to slowly drift Polaris into its designated slot on the reticle pattern.

The necessity of precision

A precise alignment allows the motor drive to counteract the rotation of our planet by turning only one axis. If your alignment is sloppy, stars will slowly drift north or south in the eyepiece, forcing you to constantly tweak both axes to keep the target centered. While some observers use simple equipment to seek out bright comets, tracking these targets at high power requires a properly aligned mount to keep the details sharp. For visual observing, a rough alignment that keeps an object in the field of view for fifteen minutes is perfectly adequate, but astrophotography demands near-perfect precision to prevent star trailing.

To achieve the high level of accuracy needed for long exposures, astrophotographers often employ a method called drift alignment. This technique involves monitoring a star near the celestial meridian and equator, observing its drift over several minutes, and adjusting the azimuth. Then, a star near the eastern or western horizon is monitored to adjust the altitude. While time-consuming, this iterative process bypasses the limitations of the polar scope and compensates for minor mechanical imperfections in the mount itself.

Daytime alignment techniques

Aligning an equatorial mount during the daytime presents a unique set of challenges because Polaris is completely invisible. This scenario is common when preparing the mount for daytime sunspot counting, where you must track the sun across the sky for hours. To do this safely, you must use a reliable compass, correcting for the magnetic declination of your specific location to find true north. Before pointing any optics at the sun, ensure your front-aperture filter complies with the safety requirements of ISO 12312-2 to protect your eyesight. Once the mount is oriented to true north and set to your local latitude, the tracking motor will keep the solar disk centered surprisingly well throughout the afternoon.