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c8 repair – dec lock June 4, 2009

Posted by aquillam in Astronomy, telescope maintanance.
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This post is one in a series about C8 repair and maintenance.  You should see the introduction and disclaimers before trying anything here.

When mounted on an equatorial wedge, the telescope rotates in right ascension (RA) and declination (dec). The telescope needs to move freely to align it, but once aligned, you don’t want it to move too easily, so there are locks in both directions.  There are also fine motion controls for both directions. Below are instructions for adjusting the dec lock.  You might also want to check out the instructions for opening the base, dec fine adjustment, and the RA lock.

The dec lock is on top of the left arm (it you are standing on the side of the telescope with the RA lock, which is the normal side of the telescope to be on). The dec lock is a tension ring. The lock lever simply tightens the ring.  When fully locked, you will still be able to force the telescope to move.  That isn’t very good for the ‘scope however, so you shouldn’t do it intentionally. Unlocked, the lever is tangent to the dec scale (see image below), locked it is parallel to the arm.

unlocked dec lock

unlocked dec lock

The lock lever basically pulls the two sides of the tension ring together (see the zoomed in image below.) The cap on the left determines how far the lever will pull.  Note the cap may just be a nut on some ‘scopes. The nut in the middle prevents overtightening, and helps push the ring open when the lock is unlocked. properly adjusted, pushing the lever up closes the tension ring on the middle nut and the ‘scope won’t move easily. Moving the lever to the open position will push the tension ring open and the ‘scope will move freely.

The correct adjustment should have the cap, left side of the tension ring, a washer and the middle nut fairly tight together.  Then there should be a small gap. Next the right side of the tension ring, another washer and the lever should again be tight together.

Detail of dec lock

Detail of dec lock

To tighten the lock, first try moving the middle nut to the left.  It should be up against the left side, as in the picture above (the nut is up against a washer on the left side, and there is a small gap on the right side.) If that isn’t sufficient, loosen it slightly, and tighten the cap, then tighten the middle nut again. One tip: it may be easier to hold the cap and turn the lever.

If it is difficult to unlock, loosen the cap until you can push the two parts of the tension ring apart enough to unlock. Once you have the gap big enough, move the middle nut to the left until it is up against the tension ring. Moving the lever to the unlock position should now open the ring enough to unlock the dec motion.

If the lever won’t hold in the locked position, it may have become worn.  You can try turning it upside down (don’t forget to adjust the cap if you do that). If that doesn’t work you may have to take the lever off and file one side to make it more flat. It shouldn’t take much to flatten it out enough to stay locked. Also, make sure it is clean and free of grease or anything that can make it slippery.

One other possiblity, especialy if you have a lot of equipment on the ‘scope, is that it needs to be balanced. No matter how perfect the lock is working, it won’t support 10 ponds of imaging equipment off the back.  You’ll need some extra equipment for that, so that’s a post for another day.

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