My 10" f/10 LX-200 just had its first birthday and to celebrate I decided to
do a very minor mechanical teardown to see if I could get to the bottom of
what I perceived to be some looseness in the declination axis. Also I
noticed recently while set up on the superwedge some retrograde motion when
using the dec drive that I had never noticed before, but which only seemed
to show up when pointed in certain directions. I felt my tracking and
pointing was OK, but not as good as it should be compared to reports from
other owners. In some cases, when slewing perhaps 60 degrees, I'd be off
target by up to a degree. Not very good, but I assumed I just hadn't done a
careful alignment. Sometimes accuracy seemed to be much better, but not very
often.

I should mention at this point that since the first day I set up my LX-200,
I noticed a "thud" or "clunk" as I moved the OTA up and down in declination.
If I loosened the dec clutch and rotated the OTA starting from pointing
straight down, somewhere just above horizontal I could feel, see, and hear a
"thud". Having no basis for comparison, I thought this was "normal". I'm
sure most other owners are probably laughing at me by this point, but I
didn't know any better. I probably thought this was the "mirror flop" that
I'd read so much about.

Before I started disassembly I read up on Doc G's advice as well as all the
other advice I could get my hands on regarding the dec drive. I'm a bit
mechanically challenged, so I proceeded very slowly and carefully. I
removed the dec housing, then put the clutch knob back on and tightened it.
Looking for any looseness between the worm and the bull gear while rocking
the OTA slightly, I could see none. But I was definitely moving the OTA
slightly, perhaps one or two degrees, maybe even more. So where was the
looseness?

Finally, I noticed that as I continued rocking the OTA, there was a definite
motion between the OTA tube and what I'm going to call the OTA support
bracket (not the right term I'm sure). This is the black bracket that is
bolted to the OTA and is connected to the dec bearings, I presume. I could
see very considerable motion (flexure) taking place between the OTA support
bracket on the dec motor side and the OTA itself. This is what was allowing
the OTA to rock even when the clutch was tightened. Definitely did not seem
right to me (duh).

When the OTA is pointing straight down, there are three allen-head bolts in
this support bracket above the dec bearing, two larger, one smaller. I put a
wrench on the smaller of the three and tightened two full turns!! This baby
was loose! I could actually see the support bracket snug up to the OTA as I
tightened. I did not tighten for all I was worth, just to firm tightness.
Next I checked the two larger bolts on that same side and got nearly one
full turn on each of them. Way too loose as well! Of the three on the other
side (opposite the dec drive motor) the two larger ones got about 1/2 turn
each and the smaller one on that side was already tight.

Well, immediately the "thud" was gone when I rotated the OTA in declination.
Also the amount of play I get when I try to rock the OTA with the clutch
tightened is much less, almost nothing compared to before. Unfortunately I
'm going to be clouded out for the next couple of days so it will be a
little while before I can see how much this improves my pointing accuracy
(and the retrograde motion problem). But I have to believe that the amount
of flexure and looseness between the support bracket and the OTA that I had
before was a huge contributor to my accuracy problem.

I've gone on long on this because I haven't seen this particular problem
posted before. If I had thought to check this before, I could have found out
in less than one minute (without disassembling anything) that these bolts
were loose. All you need are two proper size allen wrenches to check. I'd
definitely recommend anyone having pointing accuracy problems to check those
six bolts before you do anything else.

And now I know that my LX-200 is NOT supposed to go "thud".

I'd be very interested to know if anyone else has come across this problem.

Clear skies,
Kevin Wigell

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