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Replacing the Declination Bearings
on the 12" LX200

I decided it was time to replace the bearings in the 12" LX200 which I had a year earlier donated to the Madison Astronomical Society. This telescope has been living in a permanent building at the MAS dark site for two winters and a summer. In Wisconsin this is an appreciable time subject to the complete temperature and and humidity ranges extant.

The telescope has been working perfectly in every way with the exception that the declination drive failed. This failure is described elsewhere in this web site.  After inspecting the drive I  found that the gear on the end of the worm shaft had worn its key way and was able to move several degrees on the shaft.  I replaced the broken drive with a new drive which I purchased from Meade.  The telescope then came back to its normal operating condition and pointing accuracy, which was actually quite good.  I felt that the original source of the failure was that the declination bearings were causing rough motion of the declination mechanism.  The Hart bearing replacement suggestion came along at this time.  I got experience in doing the replacement by replacing the bearings in my own 10" LX200.   This was convenient to do since I had the telescope in my basement laboratory for other tests at the time.

Doing the second replacement was a very easy job.  I dismantled the 12" telescope at the observatory in about 40 minutes.  The most difficult part of the operation was again removing the declination clutch screws.  They were both very tight and pre-damaged.  They came out but with a medium tough struggle.  I took the fork arms home, mounted them in my mill and bored out the bearing holes in about 30 minutes.  The new needle bearings were inserted and held with medium loktite.

The fork arms were returned to the observatory and the entire telescope put back together.  This took about 45 minutes.  The results were even more pleasing to me than those for the 10" telescope.  I feel that the amount of improvement for the 12" was much greater than for the 10".  This is, I think, because the basic mechanism, mechanics, of the 12" is much more marginal than for the smaller 10" telescope.  The weight of the 12" especially with the accessories that this 12" carries is great enough to stress the drives provided.

The 12" carries the Meade dew hood which is quite heavy and placed well to the front of the declination axis of course.  There is also a Celestron C-5 in Losmandy rings and with a Losmandy rail which adds quite a bit of weight, fortunately very near the declination axis.  On the back is a JMI focuser, a 2" diagonal and 2" eyepiece and on occasion an ST-7 CCD imager.  In order to balance the accessories, especially the hood, it is necessary to use  7 pounds of Meade weights as close as possible to the back plate and a Losmandy 2D rail and weight set consisting of 7 1/2 pounds of weights a maximum extension from the optical tube.  (The Losmandy 2D rail was extended to fit the 12" telescope optical tube.)

Before the bearing replacement, it was almost impossible to tell when the tube was balanced because of the excessive friction in the declination bearings.  It is now very easy to balance the tube using the two position method because it is very clear when balance has been reached.  In addition to much better balancing, the Slew setting is always set to about 4 degrees per second instead of the maximum slew rate.  I have always felt that the maximum slew rate is too high for the capabilities of the very small drive motor and gears used on these telescopes.  There is really no reason that the telescope needs to slew as fast as it will at maximum setting.  Unfortunately, the computer does not remember the slower setting when turned off.

I am very pleased with the bearing replacement and now feel the telescope is much less fragile and will serve more hands without damage.  I will report on the success of this modification in the coming months.  In over a year now, there have been many reports of successful modifications of LX telescopes using this information.  A few concerns have related to some specific telescope where slightly different use of thrust washers and final adjustemnts were required.

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