In March of 2000, an important post appeared on MAPUG-Astronomy
which describes some experiences with carrying out the dec bearing modification.
This addendum is an edited version of two posts on this topic from Neal Barry.
Neal has modified three LX scopes and can be considered one of our growing clan
(The following is a paraphrase of his comments by Doc G)
1. I had a difficult time getting the under-side plate of the declination clutch off. I was able to get the two screws out (surprisingly, without heating to soften the Loktite) I then used a small wheel puller that pushed off of a 1/4-20 bolt threaded into the clutch knob hole to pull on the clutch plate. I tightened it up real well - nothing happened. I waited a while and tightened it a far as I felt was safe - still nothing. Hmmm... For the heck of it, I got out the ol' propane torch and heated the center of the plate for about 45 seconds. Still nothing! As I walked away, I heard a terrific bang - it broke free! Two of the three LX telescopes I've modified have had some sort of really nifty adhesive that bonded the clutch plate to the declination shaft in addition to the two screws. (Ed. Apparently some significant pulling has to be done on some of the telescope clutch plates to get them off. On the telescopes I modified they came off easily.)
2. I used an SCE2012 (McMaster p/n 5905K29) needle bearing with the addition of a PI162012 (McMaster p/n 7929K56) hardened steel inner ring on both axes. I found with the first telescope that I modified that using a needle bearing alone would ultimately put indentations in the original LX200 case aluminum shafts when carrying a heavy, but reasonable, load. As the indentations 'grew' it became increasingly difficult to achieve accurate pointing. (Ed: This is a very important bit of new information. It might be wise when doing the modification to go to the steel sleeved bearing parts.)
3. Through experience I found that I needed to bore out the LX200 arms significantly undersize in order to eliminate the lateral slop. In the case of the bearing combination above, the SCE2012 is approximately 1.505" and is designed for a press fit into a 1.50" hole. I had to actually bore the arms to 1.496-7" to achieve a compression fit with approximately 0.0005" lateral play between the inner ring and the needle bearing. (I used Loctite to fix the ring to the declination shaft..) This might seem like nit-picking, but the difference in pointing accuracy was quite pronounced. With any slop, the entire OTA would 'flop' over as the telescope rotated in RA.
4. When you reattach the arms, you need to provide sufficient preload
to the declination axis. (between the arms) Normally one arm is
already locked in place (see Doc G's main article) and then you accurately
tighten up the other arm to a predetermined position which should 'pinch' the
OTA between the two arms. The shoulders on the two plastic bushings that
were replaced by the bearings contribute approximately 2 X 0.042" (0.084") to
the declination axis preload. I used a single additional 0.031" thrust washer
(McMasters p/n 5909K49 on the arm that carried the declination drive gear and
two of the same thrust washer on the opposite side. This results in slightly
more preload than comes from the factory, but I have found that a little more
does not hurt and too little can contribute to pointing errors.
(Ed. My experience has been similar. The amount of preload necessary becomes apparent when the arms are re-assembled and tightened. Tighter is better that allowing any slop.)
With everthing fitted tightly, it is wise to align the arms accurately. The use of a 1" precision ground steel bar is helpful. Detailed instructions for re-assembling the telescoe arms is given in the main article and should be followed carefully.
Overall, I would recommend this modification to any LX200-12" owner who has the necessary mechanical savvy to carry it out and needs the improved pointing accuracy and/or has a heavy load of accessories on the OTA. In all three of the reworked LX200 telescopes the pointing accuracy improved to better than 2 arc minutes across the sky (with mirror flop eliminated) On my own scope, on which I've spent a little extra time, I've achieved better than 25 arc second pointing accuracy with the help of T-point. This is quite good for a mass-produced telescope.
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