c8 repair – opening the base part 4: the RA lock

This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here.

This is part 3 (obviously.) You need part 1: opening the base before you can do the things on this page. That’s also where you’ll find the links to the other things you can do with the base open.

RA fine motion adjustment

The RA find adjustment knob simply turns the gear directly underneath it. The gear should be greased (carefully). Problems with the fine motion control usually come from sticky grease or the RA lock, although you might also check for broken teeth. If turning the knob doesn’t turn the little gear, check the set screws that hold the knob to the shaft.

RA Lock

The RA lock is a metal plate. When the lever is turned, it forces the plate down and it presses on the gears to stop them from turning.

The RA lock plate and RA fine adjustment gear. The RA fine adjustment knob is visible at the bottom, and the edge that needs to be greased (shown here with some old black grease) is near the top of the image.

The most common problem with the RA in a telescope that has been opened is grease on the lock.  Be sure to keep the grease well away from the plate and off the top of the gears.

The plate will experience some wear, especially if the telescope is stored with the RA lock in the locked position, or if the telescope is repeatedly forced to move while locked. If the telescope is repeatedly moved when locked, the gear can actually carve a groove into the bottom of the plate so that it won’t lock anymore. If it is left locked all the time, the pin can carve a hole in the top, and the lock will cease to lock.

Top side of the plate, showing light wear on the top.

A simple fix for that is to simply turn the plate over.  If it is very worn (or worse, warped), you may need to machine a get a new one. I recommend working for an astronomy department with a machinist. That’s always worked for me. (Actually, what I really recommend is not turning the telescope with the RA lock locked so you don’t have to replace the plate…)

From here, you may want to go on to:

• opening the base part 1
• opening the base part 2: the bottom and motors
• opening the base part 3: the gears
• opening the base part 4: the RA lock