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NexStar GPS Azimuth Motion Rough
Cleaning the Roller Bearings

On the NexStar 8/9.25/11 GPS mount, the upper base rotates on three roller bearings consisting of steel wheels riding on an aluminum track in the lower base.   The inside (toward the center of the mount) edge of the wheels needs to turn slower than the outside edge of the wheels (and incrementally in between) for both edges to actually be rolling along the aluminum track. That is physically impossible; some part of the wheel will always be sliding along the aluminum, not rolling. For example, if the outside is rolling, the inside is being forced to move faster than it should and it is sliding on the track. Steel sliding on aluminum will scrape a little aluminum off the track as time goes along and it accumulates as rough spots on the wheels and track resulting in a rough "rumble" that you can feel when you rotate the scope with the clutch disengaged.  This also inhibits smooth tracking and manual slewing with the hand control. The engineers came up with a bearing that is rock solid, even on a wedge, but they neglected to think through this little detail.

Both the wheels and the track should be smooth and clean.  Fortunately, cleaning the wheels and track is not difficult.  Release the altitude clutch, move the optical tube perpendicular to the fork arms, and reengage the clutch.  Using a 3.5mm or 9/64 inch hex wrench, completely loosen the five screws securing the plastic base cover - the cover with the Aux, Auto Guide, and PC ports.  It is not necessary to remove the screws as we will only be lifting the cover, not completely removing it.  If you do remove the screws, note that the one in the back is shorter than the rest.

Carefully lift the base cover.  Wires run from the cover to the base and only allow the cover to be raised about 2 inches (5cm).  Put a rolled-up sock in both gaps between the cover and each fork arm to hold it in place.  Dampen a lint free cloth (not tissue!) with alcohol.  Do not make the cloth so wet that you can squeeze liquid out - the excess alcohol could rinse away the grease on the bearings.

Start by cleaning the wheels.  Release the azimuth clutch.  Hold the damp cloth to one of the wheels and rotate the base.  Be careful to prevent the cloth from catching between the wheels and the base.  After you clean the wheels, clean the track.  Find the largest opening leading to the track and clean the track while rotating the base to gain access to the entire path.

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Michael Swanson
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