Dec Axis, Backlash, & Motor Issues (Classic)  

       

Continued on:    

Subject: Dec Axis Fix URL

From: Kevin Wigell <kevinkwastronomy.com> Date: Nov 2002

I have added a page to my personal web site devoted to the Dec drive assembly. There are lots of photos and descriptions of the process of taking it out, making simple adjustments, and putting it back in. Doc G has a superb discussion of the Dec drive on his web site, however my page contains much less technical information than Doc G's. My site is aimed more at someone like myself, who might want to know more about his/her dec drive, but who may not be a mechanical wizard.

The Dec drive page is here:
  <http://www.kwastronomy.com/LX200_Dec_Motor_Assy.htm>

Subject: Declination Assembly Adjustment  

From: Larry Smith

The Dec. assy. is not necessarily the only cause of play in the Dec. axis. be sure to check the side plate on the OTA as well. Also, check the large Helical cut gear on the Dec. axis. Be certain these mounting areas are secure before going on to the Dec. Drive components. By the way, if anybody finds that an OTA side plate mount has become loose, (not real likely), the scope will have to go back to Meade for optical bench alignment, as it would be difficult to do this setup without the necessary equipment to assure parallelism between the fork sides and the optical axis (pointing in R.A. could be degraded). Moving on to the Dec. assembly, there are several possible areas of excess motion in the mechanics of the assembly.

Basically these are: *Worm Gear End Play *Worm Gear Engagement *Worm Gear Drive Assembly Adjustment. First, restrain the OTA by some means so as to prevent it from swinging down of its own volition. When we disengage the Worm drive assembly this is likely to happen. Next remove the Dec. clutch knob and the Dec. drive assembly cover. Now loosen the two (2) Allen head screws that hold the assembly to the inside of the Fork, (just enough to allow an up/down sliding of the assembly).

Next, lower this sliding adjustment all the way down, (Dec. assembly towards tripod). Now with your finger, push the worm mount down (towards tripod). The Worm mount should move up by itself when you remove your finger, this should move up freely with no apparent inhibitions (you can follow it LIGHTLY with your finger to confirm an upward free movement). While the assembly is in this "lowered" position, we can check the Worm Gear end play. You will need a flat blade feeler gauge set (the kind that looks like a folding pocket knife with a lot of blades). Power on the Scope and move the Declination "North" slightly, via the KEYPAD, (this loads the end play in the FIRST direction).

Now, find a blade in the feeler gauge that fits in between the right end of the worm and the end bearing mount, (we are looking for a slight friction slip Þt here, not tight & not loose). When the proper blade Þt is found, (you MIGHT find that one blade is too tight, & the next closest size,{ .001" difference), is too loose. If you encounter this measurement difficulty, do not give up! It just means that the gap is a value somewhere in between the first and second blades. (WE ARE ONLY TRYING TO ESTABLISH A REFERENCE AT THIS POINT).

Once we find this reference, WRITE IT DOWN! Now, move the Declination "South" slightly, (again with the keypad), and repeat the above "gap determination" procedure. If you find that there is ANY difference in the two measurements, then there is TOO MUCH worm "end play". At this point you should remove the Dec. assembly and send it to Meade for re-adjustment, of course enclosing a note stating the complaint. This is a very un-likely problem unless your scope has had an awful lot of Declination movement, or possibly had a signiÞcant amount of Dec. motion while being loaded with cameras, counter weights etc., or possibly being operated with a signiÞcantly out of balance load, i.e. camera, off-axis guider, eyepiece and no counter balancing. The "End Loads" on a Worm Gear are quite high (not just in this system!). As I mentioned before, this is not an area that is prone to early failure, but is a wear point that could cause pointing in-accuracy. From the sound of Todd's description, this is not a worm gear end play problem. now that we have completed this exercise, let's put the Dec. assembly back into adjustment, (oops! I forgot to mention that if the spring return test we started with fails, you will have to try to determine what is causing the interference).

OK, now move the two mounting screws that hold the assembly to the fork, in & out until we have a condition that allows us to move the assembly up & down (**GENTLY** a flat blade screw driver as a lever). Adjust the assembly UP into engagement with the Dec. drive gear so that about 70 - 80 percent of the spring loading travel is taken up, (compression). Now here is a funny part ( I'm not grinning about this), the small aluminum gearbox on the left end of the assembly has a little rectangular foam/tape cover on the its topside. (mine, and another I have worked on, was green in color). If we do not adjust the assembly UP into the drive gear with the left side of the assembly tipped DOWN slightly, the Dec. drive gear will rub on this cover, and chew off little pieces of it. Not good!! this will not only possibly peel the cover off the reduction gearbox, exposing the contents (including the optical encoder assembly), but a large enough chunk COULD come loose and become jammed between worm and Dec. drive gear. So before putting the cover and knob back on, run "North" and "South" a couple times to assure oneself of clearance between drive gear and reduction gearbox cover. (It only needs to be a very slight clearance, it's not going to go anywhere.)

While running the scope "North and "South", watch the worm gear for excessive up/down motion, if there is such, you probably don't have the worm adjusted into the drive gear sufficiently. Time to go thru the procedure again ! In so far as adjusting the end play of the worm gear goes, most users DO NOT have the necessary tools (such as Dial Indicator), to do this properly. Sure, you could take a large screw driver and start twisting that big screw on the end of the worm gear bearing mount until you thought you had it right, but there is very little chance that the adjustment would be correct. Most likely you would make it TOO tight in trying to remove the end play, and this procedure will probably cause eventual, if not immediate damage to the plastic gears in the reduction gearbox. The worm gear MUST be disconnected from the gearbox before end play can be safely adjusted!! Therefor my recommendation of returning the assembly to Meade Instruments.

Subject: Dec Backlash or Retrograde Motion Explained 

From: Doc G, Date: Oct 2003

To understand the backlash phenomenon, which is very subtle mechanical effect, you need to understand the difference between a worm wheel and a spur gear. The spur gear has a flat surface presented to the worm. The worm touches the surface of the gear at only one point. This is the type gear used by Meade for reasons of cost. The true worm wheel is a hobbed gear which fits the worm along its circumference. This is the more typical gear used in precision mounts. This type of gear is called a throated gear.

The cause of the retro motion is that there is a bit of stiction between the worm and the gear when it goes through a reversal. That is the surfaces are momentarily static and static friction rules. When the worm turns, it bends the main large gear (actually the joint of the gear with its shaft--see below) a slight amount in the direction the worm surface is trying to move. An analysis of the resulting motion of the main gear shows that it moves in the wrong direction briefly and move in the correct direction when the worm starts to slip on the gear surface. When the worm is slipping on the main gear, dynamic friction rules. Dynamic friction is much less than static friction.

The reason some worms and gears do not show retro motion is the pressure setting of the worm against the worm wheel happens to be just right. In a precision system, the throat of the worm wheel, the hobbed gear, prevents the lateral motion of the main gear because it is locked into the worm because of the shape of the contact surface. That is why precision mounts use worms and true worm wheels. This all becomes obvious when you look at the figure and think about it real hard.

Explaining this retro motion in words is terrible hard. I have another way of looking at it which may help. I will try.

Think of the worm as a ramp. It is actually a ramp wound up into a spiral. Draw this ramp from the side so it looks like a simple straight line. That would be the worm rolled out on a flat sheet of paper.

Now, because the gear is a "flat gear" which is not hobbed, the gear touches the ramp at one point. Now think of moving the worm (the ramp) in a direction to the right. This would push the contact point along the ramp and move the contact point say away from you. This would be the result if you had tilted the ramp from the northwest to the southeast in your first drawing of the ramp.

If the contact point remained exactly on the same vertical line all of the time, the contact point would simply move away and toward you as you slide the ramp right and left. This is perfect action as is desired.

But now assume, as is the case with the Dec gear, that there is enough stiction to force the contact point in the direction of the motion of the ramp, that is to the right, a small amount. When this occurs, the contact point is also dragged toward you a bit. That is because the ramp is moving along its surface line from right to left. This is retro motion.

Finally, the stiction is broken, the contact point moves back to its normal vertical position and the contact point moves away from you as it is supposed to do when the ramp is moved to the right.

Thus, the retro motion is due to the flexing of the Dec gear. This does not happen with the RA gear because of the way it is mounted so it cannot flex. All of the above takes place even if the rest of the mechanical system is perfect. Note that this flexing of the gear cannot take place with a hobbed gear because the hobbed gear is locked into the circumferential surface of the worm.

I hope this works for you while I try to get some diagrams to you in some way. I may have to write a document in word and convert it to html and then transfer it to my web site. That will take me a while.

I believe I have a few comments and clarifications to make about the Dec gear design that will clarify the following suggestions. I will take them both at one time and insert my comments in their text below:

> If the Dec gear were made to be less flexible, would this improve the
> situation. Would a brass or a steel gear be better? Could a "stiff"
> plate be attached to the outside of the existing gear to reduce flexing.
> Is it too impractical to consider having a replacement gear made that
> is a true worm wheel?

I am afraid my wording gave a wrong impression. It is not the actual gear that is flexing or distorting. It is the gear moving laterally at its edge because it is "rocking" on the end of the declination shaft. The way the gear is mounted on the 1" declination shaft is a very weak point of the design. The gear is simply placed on the end of the shaft and bolted in place with two small screws. These are the famous screws which have to be heated with a torch to remove them. The gear actually rocks on the end of the shaft because it is impossible to make this joint tight enough with two small screws no matter how tight you make them. The mounting of the gear could be improved by machining it to fit the shaft, adding strength at this point, splining or something like that. This joint is also, by the way, a source of looseness in the coupling between the rotary motion of the gear and the OTA which occurs when the screws work loose or are strained a bit. Many have experienced this problem.

> I read somewhere about a technique to slightly angle the worm shaft
> relative to the plane of the worm wheel to reduce play when the
> direction of a worm is reversed. Could the drive assembly be shimmed on
> one side to effectively rotate the worm axis to better "fit" the
> existing flat worm gear? A measurement I made on the worm indicated the
> drive assembly would have to be rotated between 5 and 6 degrees to get
> the worm to "seat" into the worm gear. The contact would probably still
> be a single point, but the angle of the contact would reduced.

Now this is getting into "deep thought." I have not tried the techniques mentioned here. I have my doubts that it would help much. Remember that the worm surface is already, supposedly, at the correct angle to engage the teeth on the gear which are also at an angle. While you do not get the intimate meshing here that you do with a hobbed gear, the two surfaces are at least osculating. I was a bit extreme to call it a point contact. I think an osculating contact would be a better description. So there is a small contact surface and as intimate contact as one can expect in this case. As a consequence changing this angle may do little good. Still this retro-motion effect is so small and depends on details of the contact that you might luck out trying this.

Subject: Dec Backlash, Some Fixes   

From: Robert Preston

Momentary motion of a star in the opposite direction to the direction that is intended was discussed on MAPUG a number of months ago. Someone coined the term "retrograde backlash" to distinguish the phenomenon from ordinary backlash. As I recall, the exact cause of the problem was never pinned down. I was thinking about the puzzle again while trying to help someone solve a problem with ordinary backlash. I think I know the main culprit and one possible fix.

DEC motion can arise from two different worm motions. The first motion is the one that the LX200 designers would like to be the only motion: rotation of the worm causes the worm's teeth to advance the wormwheel (in that case, the axis of worm rotation and the axis of wormwheel rotation, or the DEC axis, are absolutely rigidly fixed relative to one another EXCEPT for the slight up-and-down motion allowed by the spring-loaded worm carriage). The second motion is the one that causes problems: several different mechanical failures can make it possible for the worm to slide in and out along a line parallel to the DEC axis, so that the angled worm teeth can move the wormwheel without any rotation of the worm (this defective motion follows from the fact that the worm teeth necessarily have a pitch angle).

The main culprits (I suspect) are the bearings in the hinge of the DEC motor assembly. This is the hinge that allows the (normal) slight up-and-down motion of the worm against the wormwheel, loaded by that little coil spring. I have taken the hinge apart, and I found that there are two bearings in it, one at each end of the hinge. Each bearing consists of a steel ball that is gripped by two hardened steel Allen-head set-screws. In the situation I mentioned above, the hinge bearing that is closer to the front of the OTA (the corrector plate end) had become damaged by excessive force on the OTA during transportation of the scope with the DEC axis locked. The person reported that the hinge had noticeable "wobble" in it, and the scope had developed a DEC backlash that made CCD work impossible. After I explained the construction of the hinge, he tightened the one bearing and the scope was as good as new. I think the original bearing set-screws probably had a high spot that had been functioning OK as a bearing surface, but the high spot had been flattened by the unusual force on the OTA, allowing slop in the hinge.

It can be calculated that a tenth of a millimeter slop in the hinge will translate into 180 arc seconds of DEC slop in the sky (backlash or GOTO slop or whatever). People with DEC backlash or slop (retrograde or standard) might check that hinge for tightness: it should have absolutely no slop (but still be freely movable by the slight pressure of the coil spring). If it needs to be tightened, be sure to use an exact-fit Allen wrench in good shape, because the set screw is locktited tightly and a bad-fit wrench would strip the head and then you'd need a machinist or, cheaper, a new DEC assembly. The hinge bearing that is on the end of the hinge that is closer to the eyepiece can be accessed (I think), by removing the two set-screws and the ball from the other bearing first and then inserting the Allen wrench into the resulting hole to get to the head of the hidden Allen screw (the alternative is to gain access to the other side by taking the gear-box apart and removing it, but that can be a scary task, although it is do-able). In the situation above, it was lucky that only the bearing at the more easily accessible end needed to be tightened.

This message is already long, so I won't go into the analysis that says that retrograde DEC backlash (as well as the typical backlash) would also be caused by a loose hinge (or by any other non-rigidity between the axes of the worm and wormwheel apart from that permitted by a well-adjusted hinge).

Subject: Retrograde Backlash - Another Clue  

From: Robert Preston

Both Howard Anderson and I have studied this problem intensively (independently) using laser-pointer reflections from the motor assembly. Some of Howard's comments are posted on his web page. The bottom line for me was that I eliminated retrograde backlash completely simply by unscrewing the motor assembly slightly (two allen-head screws) and then tightening it down again, in almost the very same position it had before I loosened it. It seems that the allowable slop in the mounting holes of the motor assembly had allowed my motor assembly to be located in a position that caused retrograde.

Ric solved this (or a regular backlash, perhaps he will comment further since I can't remember) by removing one of the two springs (on his scope) that push the motor assembly and worm against the wormwheel. It appears that some scopes have two springs there, some (mine being one) have only one spring. So your comment below is right on the mark.

The precise angle and position of the worm relative to the wormwheel, as affected by the slop in the motor-assembly-mount, needed to be optimized in my case. The laser was not really necessary, trial and error in loosening and tightening the two screws would have been sufficient.

Piper is being pessimistic about retrograde being inherent in the design. My scope now has no detectable retrograde, under a variety of loading conditions. I suppose Piper might have meant that, with the design used, it is not possible under production conditions to get the retrograde in all scopes small enough to be unnoticable. I could easily believe that, since it is a very touchy system. But I have the evidence of my own eyes to prove that retrograde can be eliminated, at least on some scopes.

Recently, I describe a more recent finding in a scope that had excessive ordinary DEC backlash. Slop in the motor-assembly forward hinge (induced by banging the locked OTA during transportation) had caused that problem. The owner and I worked by e-mail to diagnose and solve the problem by tightening the hinge allen screw a pinch. I suspect this same hinge may be part of the retrograde problem in some scopes, but I don't have any good evidence yet (no one to my knowledge has tried to fix retrograde by tightening a loose hinge bearing - and, knowing the touchiness of the system, I'm certainly not going to try to prove the point by loosening my scope's hinge). If anyone wants to look into this, the details of the hinge bearing construction are posted in my note a month ago (see preceeding note).

I had an old dead dec assembly that I tore into to look at this bearing: The screw that regulates the worm endplay is actually a screw/bushing (they loc-tite that thing in place at the factory). Its inner face is a cylindrical face that bears down on what looks sort of like the roller-cage of a roller-bearing. Rather than wear (or a loosened screw) accounting for the change you observed, I suspect that whoever assembled the thing did not originally press the bearings tightly into their final seating positions before applying the loc-tite. Over time, the roller bearing on one end or the other probably slipped away toward its proper seat, leaving slop in the end-play. Finally the end- play got large enough to force you to investigate. The bearings might or might not be properly seated now, after your adjustment, so that one could slip again and loosen the end-play: but now you know what to do if that happens.

Subject: Dec Backlash Cures  

From: Burton Whicker

Factor #1: The major culprit was the pivoting axle. It is an aluminum axle about 7/16" in diameter and about 3" long with "cupped" ends. The mounting plate for the whole Dec assembly has a bracket arrangement that holds the axle. The bracket has two Allen head screws with "ball" ends that form the pivot bearings for the axle. On my scope, the outside (toward the front of the scope) ball was so loose it was barely holding the axle in the bracket. It's a wonder the whole thing didn't just fly out of the housing. The ball end screw was extremely tight (possibly due to something like Loc-tite added to the threads). I had to completely disassemble the unit and put the mounting plate in a vise in order to loosen/tighten the screw. However this cured 90% of the problem.

Factor #2: The assembled dec drive has two slotted mounting holes that allow for tangential adjustment of the worm gear with the drive gear. This alignment was also off.

Factor #3: The inner clutch disk on the big declination drive gear was loose. This was keeping the actual scope tube from moving (for a few turns anyway) even after the worm gear had engaged the drive gear.

Factor #4: As was noted, the tensioning spring has nothing to hold it in alignment (i.e. perpendicular) to the worm gear assembly. It had slipped to the point that it was at approximately a 60 degree angle, thus reducing the force on the plate. I straightened it and hope that now that the whole assembly is properly set up it will not slip out again. I suspect it moved because my whole assembly was "precessing" so badly. I also added a second "inner" spring to slightly beef up the spring tension.

Result is that the unit seems very solid now. There's still backlash but it's of a normal nature for this type of system. I'm under 4 seconds in centering mode and virtually nonexistent at fast slewing rates.

Another Declination backlash elimination method here: <http://www.geocities.com/ginik_gr/lx200decmod.htm>

Subject: Minimizing Dec Backlash 

From: Doc G

Robert Exon wrote:
>Just an observation: After adjusting the dec gears on my LX200 I have found
>that different areas of the main gear produce much less backlash that others. If
>I suddenly start noticing bad backlash I just loosen the dec clutch and using
>the N or S buttons move to a different part of the gear.
>I just repeat this until I'm satisfied with the results. The improvement is
>usually dramatic. I attribute this to the fact that the main gear is probably not
>exactly round and some parts mesh with the worm gear better than others (or
>maybe my dec gears or still not adjusted right).

Your analysis of this problem and your solution are exactly right. I have instituted a policy for the use of the 12" LX200 in the Madison Astronomical Society Doc G Observatory that requires all motion of the telescope to de done from the hand paddle. Since this scope is permanently mounted on a pier and in polar mode, the same section of the Declination is used at all times. The drive has been optimized for this section of the gear so as to get consistent behavior.

With the permanently mounted telescope it is never necessary to loosen the clutches. Additionally, the rule is to reduce the Slew speed to 4 or slew in the Find mode. This is fast enough and saves the RA drives significantly. Finally we maintain good balance of the instrument. This instrument which is used by quite a number of persons has not failed for many months. I believe this would be a good technique for all permanently mounted telescopes.

Subject: DEC Slop Fixed (was Retrograde Backlash)  

From: Mark Buettemeier

My LX200 8" classic has always had a certain amount of DEC slop. With the clutch engaged I could grab the OTA handle and move it up and down slightly, as if something in the DEC assembly was loose. In fact I could actually here a loose clicking in the DEC assembly as I forced the OTA up and down. In RA it has always been rock-solid.

Recently the DEC slop seemed to have become worse. When in CENTER speed mode, the image would jump and I would hear a faint click as I switched between northward and southward movement or south to north.

One cloudy afternoon this weekend I decided to remove the DEC assembly cover and see if I could tell what was loose. With the clutch engaged I moved the OTA up and down to see what was moving in the DEC works. It seemed that the worm gear was slipping fore and aft on it's axis. I removed the entire DEC mechanism and tightened the worm axis with an allen wrench. (There's an allen bolt at the end of the worm axis opposite the end where the servo-motor is found.) I tightened this bolt just to the point where the slop was gone but not so tight as to cause binding. I then reassembled the whole thing and tried it out.

ALL the slop in the DEC seems now to be gone. I tried to force the OTA north and south and could feel no slop at all. Also, that night while observing I no longer noticed any jumping when I changed from moving North to South or South to North.

My only concern is this: I can understand how this might have been loose when it left the factory but I'm a bit surprised that it seemed to become worse over time. The bolt that I tightened was not loose -- how do I explain -- I had to really torque on it to affect a change in the slop of the worm axis, almost as if it were some kind of locking bolt or had been glued in place to prevent it from loosening. And yet, it seems to have loosened over time. By the way, I ALWAYS disengage both clutches before trasporting the scope so as to not put undue forces on the motors and gears. For now, the scope is 'better than new' and I'm a happy camper.

Subject: Declination Overshoot Problem --part 1 of 4  

From: Keith Graham, Date: Dec 2002

My LX200 12" classic has been experiencing a problem with declination "overshoot" when the temps dip below 25. By "overshoot", I mean that when the scope slews to a new star, it will overshoot it in declination. For example, when I slew from Betelgeuse to Sirius, the scope continues past Sirius in declination, and I will need to move the scope north to bring Sirius into the FOV. The overshoot is always in the direction of slew - i.e. if I slew from south to north, the scope will overshoot the target to the north, and I must move the scope south to put the star in the FOV.

The problem appears to be temperature related. This occurred last winter about this time but cleared up as the temps got warmer. Above 30-40 degrees, the problem does not occur. From about March through October everything works fine. Then as the colder air creeps in, the overshoot accompanies it. Last night I was out and I noticed that there was a 5-minute overshoot in declination at the beginning of my session. But within an hour and a half, I was getting as much as a 30-minute overshoot with an occasional 1-degree overshoot.

My scope is mounted on a permanent pier and the polar alignment is right on. I checked for slop in the dec gears and there does no appear to be any. However with the power turned off, there is a little slop in the manual knob before it engages the worm. I cleaned and re-greased the gears last spring, and the grease on the gears is not hardened or coagulated. Actually, the Dec mechanism moves very smoothly, and the scope is well balanced.

The RA is not affected. The star always appears right on the n-s line of the finder - it's just either north or south of the e-w line. Has anyone else experienced this and/or are there any suggestions as to how to resolve this?

----------------------------------

Subject: Declination Overshoot Problem --part 2  

From: Doc G

There are other possibilities for your problem or course. It is hard to predict what a sticky bearing might do to the system. The drive is a complex closed loop servo system which nearly defies analysis. Since I posted, I have been thinking about this problem. I have the problem of being too concerned sometimes. (G)

It is possible that this is a question of the drive platform being sticky as well. While I usually do not recommend squirting WD40 into everything, This over rated lubricant does have some nice features. It seeps into everything and sort of rejuvenates the lubricant which might have dried out or gotten sticky. WD40 is mainly kerosene which softens old lubricants.

Thus, I will suggest a modest fix that will not endanger anything. Open the dec drive side of the forks and squirt a bit of WD40 on a small brush to saturate it. Then touch this to the bearing points of the dec drive platform. That might re-lubricate things. Also then check to see that the platform has just a tiny bit of slack when it engages the high point of the dec gear.

Often just going in and tweaking around a bit will clear things up. These adjustments are partly magic I fear.

--- Original Message -----
From: Keith Graham

> Hi Doc, Indeed I have seen your write-up on dec modification. I had given
> consideration to that nylon bearing as being a possible culprit here, but
> before I delved into a major project, I wanted to be certain of the cause.
> It would seem to me that if this bearing was the cause, would that not
> cause the scope to UNDERSHOOT instead of overshoot? If the lubrication on
> the nylon sleeve "bearing" was stiff, I would think this would keep the
> scope from getting to its target, not overshoot it. Actually, I have
> virtually no sticktion in the dec axis. When I loosen the clutch knob, I can
> move the scope up and down with only a slight touch (it is actually balanced
> quite well). This is why I am so baffled. I was wondering, could the
> electronics could somehow be affected by temperature to cause the error? I
> don't think this is likely, but I am just trying to eliminate unlikely
> candidates for the problem.

----------------------------------

Subject: Declination Overshoot Problem --part 3  

From: Bruce Johnston

Keith, Let me throw out an off-the-wall idea, just in case. I've seen similar problems of going too far when a spot of grease managed to get itself down onto the encoder disk. That made the 'count' for the number of steps taken to be low, and it would go too far as a result. Of course, if for any reason, the optics electronics was marginal, it very well might cause the missing pulses, especially if something mechanical in this area was shrinking a bit under the cold.

One way to help eliminate this as a problem would be to see how far off it goes, compared to how far it was supposed to go. If it gets further off with longer distances that it was *supposed* to go, then a missing pulse very could be possible. If this doesn't follow that pattern, then it isn't very likely the problem.

Just assume for a moment that instead of it seeing the 90 slots for each rotation of the motor, it only 'saw' 89, then the more motor turns taken, the greater the error would accumulate.

Probably not, but it would at least fit the description of your failure.

----------------------------------

Subject: Declination Overshoot Problem --part 4 of 4

From: Tim Prowten <timtelescopeservice.com>

Keith, The most likely electronic culprit here is that one of the LED/Photo transistor pairs in the DEC motor assembly is temperature sensitive. If the Photo transistor output is very close to the limit on the comparator input, the comparator may not detect the transition with the result that you are missing pulses. Missing pulses makes the controller run the motor longer "looking" for the pulses lost, hence the overshoot. There is no good way to check this without an oscilloscope. If you have access to an oscilloscope you can check and adjust the photo transistor for a good output range and symmetrical output from the comparator. Contact me directly for the procedure. <http://www.TelescopeService.com>

Subject: Retrograde Motion in Dec Drives URL Link  

From: Doc G

For my comments and diagrams, GoTo:  Mechanical Concerns/LX200 Mechanical Concerns

Subject: Solving Dec Retrograde Motion... Again   

From: Bruce Johnston, Date: Feb 2004
Website: <http://www.mapug-astronomy.net/ccdastro/decfix.htm>

I wanted to let the group know about a recent experiment I did with my 10" classic. Ever since I've had it, which is more years than I even remember now <g>, I've fought retrograde motion on my DEC axis. I've tried every trick listed here in the MAPUG Topical Archives, but nothing ever really solved it for me. Even the most exotic one, which has to do with replacing the clearance set screw with a long screw and removing the compression spring, never helped.

Over the past couple of weeks, I've been modifying the gears inside the gearbox, but as an aside, I thought I'd also take another shot at attacking the retrograde motion. Based on the information passed to the group by Doc G., it seems that much or all of the problem relates to a slight lateral movement of the 'worm gear' by the worm as the worm rotates then reverses.

With this in mind, I looked closely at the mesh of the two gears and saw that the teeth on the worm gear do not bottom out in the worm, but the worm teeth do indeed bottom out in the worm gear. This is what would be expected as a result of lapping and polishing a worm/worm gear combination. However, since the worm gear is *not* a true throated worm gear, it seemed to me that this surface friction of the worm teeth against the bottom of the slots in the worm gear would be working against us in this area of lateral movement, as opposed to helping in a 'true' worm gear.

What I decided to do was, to take a thin hack saw blade and deepen the slots in the worm gear teeth. Just enough depth to eliminate the very end of the worm from touching the bottom of the worm gear slots. After going around the complete gear and deepening all 180 of the slots, I ended up with a clearance of approximately .030" between the end of the worm teeth and the bottom of the worm gear tooth slots.

Then, I polished the worm and worm gear together. In this operation I decided to get a bit exotic, so I removed the worm mount from the gearbox and made myself a home made gearbox from a small surplus motor. By messing around with the motor housing, I got it to rotate the worm at a rate of about 150 rpm, or close to one rpm of the worm gear. I did it this way so as to not be causing unnecessary wear on the gears inside the regular gearbox.

I mounted my worm gear on my lathe chuck, but never actually turned the lathe on. I just used it as a place to hold the worm gear and let it rotate. Then I took my home made gear box and worm assembly, and clamped the whole thing in the vise on my lathe. I meshed the two together and began polishing the two together. For about an hour, I 'polished' the two by using a pretty coarse mix of valve grinding compound mixed with mostly grease and let it take off most of the irregularities in the teeth. Then I cleaned it all off and switched to a mixture of various polishing compounds and either grease or oil, depending on how cold things got in my unheated garage.

I started with some 'Tripoli' compound for several hours, reversing the gear drive direction every half hour or so, then went down to a regular metal polish grit, then finally down to jewelers rouge. I had the gears running in one polish mixture or another for a total time of about 20 hours. Again, reversing direction every 30 minutes or so.

I can't say which part did the most good, but I can say that I took the finished unit out and put it on the scope, then aimed at an object 400 feet away (it's been nothing but clouds, so my test had to be on the land). Using a 4x Barlow, I switched directions over and over, and there was NO retrograde motion at all! And this was with the old compression spring back in the unit.

I should say that I made this same test before beginning, and I had definite retrograde motion, even while I was using the method where the spring had been removed and the adjusting screw set for 'slight' contact of the two gears. There was no method that eliminated the retrograde motion before having made my changes.

I can't guarantee that this method is necessary for your scope, nor can I guarantee that it'll do for yours what it did for mine, but I thought I'd let the group know of this latest...successful (for me) method of fighting retrograde motion.

NOTE: Should anyone decide to polish their worm assembly, you have to be very careful to not let the polishing compound work its way into the bearings at each end of the worm shaft. Otherwise you'll end up chewing the bearing innards up. ( I did that, but I had already planned on replacing the bearings so it didn't matter to me.)

CONCLUSION:What I did, was probably a lot more than was necessary to attack the problem. I just did both things while I had the worm gear out. Actually, I strongly suspect that it was the deepening of the slots in the worm gear that made the difference.

Subject: Tweaking the Dec Drive (classic) for Fast Guiding

From: Michael Hart

PREFACE
With numerous posts concerning Dec drive tweaking and Dec drive anomalies which I have made, some may believe the LX200 is not a good imaging platform or components are of poor design. In fact, I have had very little trouble with my 12" LX200 and the components appear pretty well made for the money. The truth is, I keep pushing the limits of this scope. Of course the LX200 is not a $7,000 mount with a $5,000 telescope, however, I sometimes forget this is so when I see the results.

I have spent less than $200 and perhaps 20 hours in modifying and tweaking. The few adjustments made have produced more results than any accessory purchase made to date. This is not meant to ignore those with mechanical or electronic problems with their scopes, rather I believe that as a production telescope, I have not seen much variance in the LX200 fit and function. This is one advantage of a mass produced item, for it is that which allows us to share common experiences and solutions on MAPUG.

BACKGROUND
One downside of a GoTo telescope is complication of the drive-train in the Dec axis needed to slew the telescope over a considerable distance in declination (or altitude in the AltAz mode). The reliable Dec tangent arm hasn't sufficient range or speed for a GoTo telescope This is unfortunate because a well adjusted tangent arm works very well. It is not unusual for a GoTo mount (including non-LX200s) to require 1-2 seconds or more for the Dec drive to reverse directions. Of course, the explanation is backlash. In the LX200 (and quite possibly in other mounts with small worm Dec drives), if we go just a bit beyond removing the worm to worm-wheel lash, friction rapidly develops between these components which may introduce retrograde. Balance, bearing friction, a well adjusted drive, and a host of other influences likely effect the degree of retrograde.

Some have completed DEC bearing mods. Essentially, the Dec bearing mods involve a method for replacing the factory nylon sleeve bearings with steel roller bearings. This is detailed elsewhere on Ed Stewart's Topical Archives and Doc G's Info Page. I believe the factory bearings are quite adequate for most scopes up to an overloaded 12" LX200 (my scope). One desirable feature with the factory bearings is a bit of tightness results in margin of safety for a typical unbalanced scope when the DEC clutch is released.

One benefit of the low friction Dec roller bearings is they allow a well adjusted LX200 and a slightly modified Dec drive to make fast (better than 1/2 second) guiding corrections in a telescope equipped with a worm and worm-wheel in the Dec (azimuth) axis. The methods described below were developed for the LX200 telescope used in the polar mode. When I say 1/2 second corrections, I mean the ability of the telescope to accept a guiding command and move the telescope in the direction required. The worst case is moving the Dec drive (and telescope) in the opposite direction of the previous correction. The RA drive is much more forgiving because we don't reverse directions, rather we slow down, stop or speed up that drive during guiding.

WHY FAST GUIDING?
Fast Dec correction rates aren't needed if one has excellent polar alignment, a good, well adjusted mount, and good seeing. At 2000 mm or less one might make a Dec correction every 5-10 minutes or longer. RA corrections may be equally as long utilizing a well programmed PEC (Smart Drive) to remove most of the PEC, (often to 5 arc seconds or better) and tweak the RA tracking clock speed. With an autoguider, one is likely to see a bit more corrections on amateur scopes since the autoguider may calculate the guide star centroid to 1/6 pixel and the typical seeing is 4 arc seconds stellar FWHM (Full-Width, Half-Maximum) values.

In addition, seeing related autoguider corrections increase as the focal length seen by the autoguider increases. Most of us can't depend on regular excellent seeing found in such places as Mt. Pinos, South Florida, and the Keys. As we increase the focal length to 3000+ mm and/or seeing worsens, it may be desirable to make faster, but smaller corrections (30-80%) of the guiding error if we are using an autoguider at this focal length. Arguably, the best solution is to drop the focal length seen by the autoguider under such conditions. We should probably not image under such conditions, but often there are compelling reasons to do so.

WHAT IS REQUIRED FOR FAST GUIDING?
The Dec drive is likely to require low friction bearings on the larger scopes (12") and quite possibly the smaller scopes. In addition, the Dec drive will need a couple of small, but user doable modifications removal of the Dec drive carriage platform spring and the addition of a manual thumbscrew for setting the worm to worm-wheel lash. I have described in detail the methods to fabricate a Dec adjusting thumbscrew on Doc G's Info Page under: Declination Drive Adjustments (without rebuild).

To achieve fast guiding on the LX200 requires rather precise adjustments of the worm to worm-wheel lash. Too much lash and higher electronic backlash settings cause jerky star movements. With less than zero backlash, friction rapidly increases and retrograde is produced. To obtain better than 1/2 second Dec reversals requires a near perfect balance of the backlash setting (the worm carriage adjusting thumbscrew) AND an electronic backlash setting of 35-45. It is likely that temperature, worm-wheel position and a host of other factors will require on-the-fly adjusting. The time required to tweak the drive is small (3-4 seconds) best performed just prior to imaging.

The key to manually setting the Dec lash on-the-fly is practice and consistency. A bit of practice at the eyepiece with a reticle at high power helps. Tweak in the right combination of electronic backlash and mechanical backlash right for your telescope. Those with Dec roller bearings can increase the electronic backlash to 35-45 for high speed (less than 1/2 second) Dec drive reversal.

At these speeds, my loaded 12" can keep up with about any typical autoguider correcting rate produced. By correcting rate, I mean the actual period of time between corrections, not the exposure time. For example, my ST-7 has a maximum correcting rate of around 1.45 seconds (1 correction/1.45 seconds) due to integration times and software overhead, etc. At the lowest exposure of 0.11 seconds.

STEPS NEEDED TO ADJUST THE DEC DRIVE FOR FAST GUIDING

1.) Assure excellent telescope balance for all anticipated imaging positions.

2.) Set the electronic backlash in the hand control for 35-45.

3.) Adjust the manual worm carriage thumbscrew:

a) Tighten the thumbscrew (move the thumbscrew to the right) until considerable resistance is felt. This firmly seats the worm-wheel in the worm.

b) Loosen the thumbwheel about 1/3 turn.

c) Tighten the thumbwheel until resistance is just felt-- STOP.

4.) Check step 3 before each image for best results.

I am able to achieve close to a 100% success at finding the optimal setting that removes lash without adding retrograde in 3-4 seconds while achieving better than 0.5 seconds Dec drive reversal rates on a heavily loaded 12" LX200 This is extraordinary performance for a worm-wheel Dec drive. As a result, I was able to shoot a 3048 mm tricolor image of M-57 under good seeing that exceeds anything I have ever done with that object.

Subject: Aligning Dec Setting Circle  

From: R. Nolthenius

>This may sound like a silly question. How do you accurately set the
>declination setting circle? How critical is it for doing the initial set-up?
> Also, how critical is it in having the power panel facing exactly to the north?

I assume you're working in polar mode. I've thought about this in the process of aligning our permanently mounted 12"LX200 on a superwedge. It is only critical that in the end you have the mount's axis aimed north. The superwedge need not be level and the dec axis wheel need not read 90 degrees (our reads about 88.9 degrees when it should read 90). It makes the initial locating of Polaris a little easier if you have accurate level and an accurate dec wheel, but not enough to spend a whole lot of time and money insuring this. Think of it this way, the wedge is only an underlying support - no more critical that it be aligned than that the ground itself be level etc. (BTW, the much-maligned bubble level on our superwedge seems to be reasonably accurate). The vital element before you proceed to setting polaris is to be sure the scopeis in fact aimed along the mount's polar axis. If you're already permanenly mounted, taking the scope off and using a level to do this, as in the Howard procedure, isn't practical. You want to turn the scope around a full 360 degrees and see that it isn't changing direction. We've tried doing this in two ways...

First: leave the power to the LX200 off.

1. Look at a small, bright, fixed object and rotate the scope, watching its reflection off the corrector plate (not easy or wonderfully accrurate) and keep adjusting the dec knob until the reflection doesn't move.
2. Finding a star close to the pole (Polaris is marginally OK; it moves 1 arcminute every 10 minutes so you'll want to move quickly), ideally only a few arcminutes away. Doesn't have to be bright - just visible. Sight the star, ideally through an eyepiece with reference markings (like Orion's guiding eyepiece) and move the scope in a full circle and keep adjusting the dec knob until the star moves as little as possible. You can adjust the superwedge azimuth to get the star centered in the eyepiece, but it's not critical. You just want the star to describe as small an ellipse as possible, even if the center of the ellipse is not at the center of the eyepiece. If your scope is like mine, differential flexure will cause the star to move even if you're right on axis. Note with this procedure that the polar axis is going to be initially aimed at Polaris, not the pole.

Now you're ready to pick up with the rest of the Howard procedure for polar aligning.

Subject: LX200 Dec. Setting Circle Adjustment --part 1 of 2   

From: Jon Brewster <jon_brewsterhp.com> Date: April, 2000

> I've discovered on my LX200 that the declination setting
> circle has moved. How is it supposed to be aligned?

I had this very same occurrence last month. Here's what I did:

1. set up in the house AltAz (no power needed)
2. point OTA at the ceiling
3. fix a laser pointer to a stable object (light fixture, book case, etc.) so that the beam bounces off the corrector and hits the ceiling
4. turn unclutched OTA by hand through 360 degrees azimuth. The beam will wander about on the ceiling
5. adjust Dec. position with manual Dec. knob (Dec. axis clutched) until beam does not wander
6. fix Dec. circle at 90 degrees
7. if there is no position where the beam does not wander you may have orthogonality problems

I found this process pretty sensitive although I did not quantify it. It does assume that the corrector is perpendicular with the optical axis.

---------------------------------------------------

Subject: LX200 Dec. Setting Circle Adjustment --part 2 of 2    

From: Steve Lindsey, Date: April, 2000

Also, in case it wasn't obvious, you need to loosen the hub that holds the setting circle so that when you follow a procedure like that below you can then tighten it back up to the correct position. It usually takes a try or 2 to get it tight on exactly the right spot.

If your mount is on a wedge you can do the same concept as below just looking at a bright star and adjusting Dec. until it stays in the same spot when rotating the RA.

Subject: Dec. Cord Damage Prevention Tip

From: James Garrard, Date: April, 2000

I keep noticing a lot of problems with that phone cord that goes to the Dec. motors. Has anybody thought to put a clip on the fork, just under the jack, so that the cord could be looped under it, about 3-4" from the plug? This would take the strain, that's applied when the scope slews, off the jack. Maybe put one on the side of the control panel also, to ease the strain on that end.

Subject: Discussion About the LX200 Classic Declination Drive and Its Repair   

From: Doc G, Date: Sept., 1997

The following are my experiences derived while rebuilding two declination drives on the Meade LX200 that I own. One is a 10" and the other a 12". I have been studying the declination drive on the Meade LX200 telescope and trying to find the reason for the large amount of lash in its operation. The instructions indicate that some lash is to be expected when changing direction; doing North to South reversals. The manual says that values of 2 to 4 seconds are normal. Since the declination drive speed in normal guiding mode is 15 arcseconds per second of time, one can correct for the delay by entering a number into the computer to correct for the lash.. Nominally, the number entered is 15 times the number of seconds of delay. This number is entered once and need not be changed. The number entered clearly corresponds to the number of arc seconds if mechanical lash in the declination drive. Technically the lash should be entirely in the gear reduction train and should be quite symmetrical. Since the maximum lash that can be corrected is 99 arc seconds, the actual mechanical lash must be less than 6.6 seconds.

Many users have found this correction does not always work. Often users have found much larger delays and delays that depend upon the position of the declination axis, the direction of the reversal, the loading on the telescope and many other elements. Hysteresis, dead zones, of up to 15 seconds have been reported. I too have found all of the above effects. The delay can vary from a few seconds to 10 or 15 seconds depending on many factors. If the delay were in the motor/reduction gear train as expected, it should not vary by much since the "winding up" of the gear train is similar in either direction. Loading effects on the declination axis are not strongly reflected back into the gear train because of the almost unidirectional transfer of forces through the worm gear. Typically it is not possible, with a low pitch worm gear, which this is, to turn the worm at all with any amount of torque on the main gear. Breaking of the gear would likely take place first. However, loading of the main gear, as by unbalance of the optical tube will greatly increase friction between the main gear and the worm. Thus with an unbalanced optical tube, considerably greater drive force through the reduction gearing is necessary.

A goal of this study of the declination drive is to determine the sources of excess declination drive lash and to eliminate them so that the drive will come up to the specifications required for the declination lash correction to work properly. This is necessary since the declination lash correction cannot be made more than 99 arc seconds which corresponds to 6.6 seconds. In other words, if the delay found is more than 6.6 seconds it cannot be totally corrected. Clearly delays, often observed, of 10 seconds or more cannot be corrected.

The main gear and its clutch mechanism are impressively well build and quite strong. It is nice to have a 146 mm dia declination gear since it should provide excellent pointing accuracy. The size of this gear is equal to that of separate mounts costing as much as the entire Meade telescope. This part of the design is excellent and very gratifying.

This study concentrates on the details of the worm drive and its engagement with the main gear, the gear reduction mechanism that drives the worm gear and the associated mountings.

Many users have discovered that end play in the worm gear mounting contributes to the reversal delay. This is certainly an important effect. With the given characteristics of the gears, a quick calculation shows that 1 arcsecond of motion of the telescope tube corresponds to only 0.355E-3 mm of axial motion of the worm. This is a required tolerance that is incredibly tight. Thus end play in the worm drive must be eliminated as completely as possible. The worm must be "snug" in its bearings and the entire drive platform must be snug in its pivot mount. Adjustments are provided for in the Meade mount via an end screw on the worm shaft bearing and a screw on the platform mount bearing that can be adjusted. Both should be tightened enough to eliminate all possible end play. The end play results directly in rotary motion of the main gear and thus in the pointing accuracy of the telescope.

I found the mechanism in one of the telescopes well adjusted (nice and tight) but the other had significant end play.

There is however another source of play between the worm and the main gear. This is the radial motion of the worm with respect to the main gear. For some reason, the worm in this design is on a "floating" platform which allows for motion of the worm radial to the axis of the declination drive. It is hard to understand why this "floating" action is provided. No other, of about a dozen worm/gear drives I have inspected, has this action. If one carefully measures the "float" action one finds that the worm can move as much as 0.5 mm radially. The amount of motion depends upon the direction of reversal and also on the accuracy of balance of the telescope about the declination axis. If the full "float" motion of the bearing platform is allowed, it results in 0.08 mm motion of the main gear edge which is 220 arcseconds of motion of the telescope tube. This is a motion ratio for the worm to main gear surface of 7:1 which seems a bit large for this type of drive. Note that these measurements were similar for two drives but may be different for other samples of the drive.

The forces upon the worm that push it partly out of engagement with the main gear are caused by the friction in the declination bearings plus the forces due to unbalance of the telescope tube. This seems to be one source of the varying delay in reversal operations. After evaluating numerous operations of the drive with different unbalance loads, it became clear that the amount of "float" is large, irregular and not necessarily repeatable. Motion of the bearing platform was measured with a precision dial indicator and varied from 0.025 mm with the tube well balanced to the full 0.5 mm with a substantial unbalance. In terms of tube motion this amounts to about 11 arcseconds with the tube balanced to 220 arc seconds with substantial unbalance. The amount of unbalance used was 0.1 Kg-meter. Again both drives behaved in a similar manner.

While the smaller of these "floats" can be compensated for, the larger cannot since the declination lash correction is 99 arcseconds maximum. There is also a time factor involved with the resettling of the "floating" platform to its new stable position. In addition, the platform takes on a different position when the tube is being driven compared to what it takes when it is allowed to rest. This settling of the platform position after ending a motion cycle causes the tube to drift of the order of 2 to 20 arcseconds. The drive mechanism seems to sort of relax after being exercised. Both drives did similar things but each to a different, and unpredictable, degree.

The computer based correction scheme would work with constant mechanical relaxation, it does not work well when the relaxation is variable and erratic. As well as being dependent upon the reversal direction, the "float" and relaxation motion was much smaller for a telescope tube that is perfectly balanced about the declination axis because then only the friction forces and acceleration forces must be overcome.

There is a small spring under the floating bearing platform that presses the worm against the main gear. If this spring is strong enough, it can keep the worm pressed properly against the main gear as long as the unbalance is small and the forces required to move the telescope tube are small. As the unbalance gets larger, the spring no longer maintains good contact between the worm and the main gear. Thus the telescope should be well balanced about the declination axis for all orientations of the fork.

The concept that the telescope tube should be kept unbalanced to keep the drive wound up in one direction is not valid in the case of the worm design. Unbalance only increases friction in the drive and requires greater drive force. Adding unbalance generally will not help nor work consistently even if the end play and pivot play have been "tweaked" out.

Unfortunately making the spring much stronger than the original causes the force and thus friction between the worm and the main gear to become too large and the drive binds. The tiny motor which drives the gear train is not nearly as strong as the motors used in many drives. This is an unfortunate limitation on any attempt to redesign and/or rebuild the drive as I have found out. Replacing the motor with a stronger one would probably require redesign of the drive electronics as well at which point the entire system would have to be redone. Thus the only thing that can be reasonably done to improve the drive is to limit the maximum "float" action of the worm platform and the motor/gear reduction parts of the drive. This can be done within the rubric of "tweaking" the mechanism.

One might wonder about the design of the mechanism in the first place. Why is the worm on a "floating" platform at all. One reason would be to keep the worm, on its floating platform and via a spring, to be held in optimum contact with the main gear. Another would be to allow for slight runout of the main gear. The main gear runout should easily be kept to under 0.05 mm on a gear with a 73 mm radius. In the case of the gear measured the runout was 0.1 mm. This is not a particularly refined tolerance but it is not very bad either. Several other worm/gear drives investigated had better tolerances and did not use the "floating" worm arrangement. It is hard to understand why this design is so much different from that normally found in these types of drives except that it allows for more relaxed parts tolerances in the main gear and declination bearing.

If main gear tolerances are typically 0.1 mm, there seems to be no reason for a "float" of 0.5 mm. In fact, there is an adjustable stop on the floating platform that limits the disengagement of the worm to the 0.5 mm observed. It seems that this adjustment could be tightened up to limit the "float" to be not more than required for the main gear runout. Reduction of the allowed worm platform motion was tried and does reduce the looseness of the drive linkage and the maximum slack allowed. To do this, the motion limiting screw needs to be raised toward the bottom of the platform. It was possible to tighten this tolerance until only 0.03 "float" remained on one drive and 0.05 on the other. This caused significant improvement in the total slackness within the drive systems.

Looseness in the drives was improved greatly. Only about 80 to 120 arcseconds of slack remained compared to 220 arc seconds without the adjustments described. Now another strange motion of the declination pointing mechanism was observed. When the motion was reversed in either direction a small retrograde motion was seen. This was finally traced to the mounting between the bearing platform plate and the gear train housing on which the motor is mounted. Unbelievably, the entire drive train/motor housing is attached to the worm bearing housing with four small bolts and a thick rubber ring or gasket (actually a small "O" ring.).. Thus the whole reduction gear train housing can move with respect to the worm bearing, and when it does, it allows the worm to rotate with it. The amount of motion on one drive was 0.5 degrees rotation of the worm. On the other it was 0.2 degrees. This corresponds to an angular motion of the telescope tube of 10 or 4 arcseconds. Before the motor drive train can move the worm any amount, the rubber gasket must go from clockwise to counterclockwise compression limits. (or vise versa for a change in the opposite direction.). This working of the rubber gasket is undoubtedly complex and may cause jerky motion of the worm often observed during reversals. First retrograde and then correct motion is sometimes observed. It is not at all clear exactly why this strange phenomenon takes place. It was however, observed to be repeatable over many reversal cycles. It must be related to the use of a rubber coupling element in the drive chain. It is a weird hysteresis phenomenon which would not take place in a linear system.

It is not clear why the gear train and motor are allowed to move at all with respect to the mounting frame. This design tactic remains a complete mystery to me. The only possible reason is to purposely introduce some springiness into the gear train to absorb excessive acceleration forces. But this explanation is hard to accept. Never-the-less, there is a springy slackness in the drive that I believe needs to be eliminated.

It is very tricky to get at the rubber "O" ring. The entire gear drive assembly has to be dismantled. This operation is full of traps and should not be attempted unless you are ready to replace a broken motor/gear drive assembly in the case that you ruin it. The drive is assembled from the inside out and at several points items are glued into place and press fitted. It is exceedingly difficult to take apart. The gear drive assembly was taken apart however, and then tightly bolted to the worm drive platform and the entire drive reassembled. The second drive was similarly reworked after the first was improved greatly.

Additionally, In both drive trains, it was found that the gear at the end of the worm shaft was not tight. In one case 5 degrees and in the other 3 degrees of looseness was found. This accounts for most of the remaining looseness and consequent hysteresis in the gear reduction system. Both gears were removed and found to have play between the plastic gear and the steel worm drive shaft. The gear, probably nylon, has a flat "keyway" on one side which simply had become distorted and no longer locked angular position of the gear to the "keyway" on the shaft. This was fixed by filling the distortion with epoxy and locking the gear to the shaft with an added lock washer under the retaining bolt. This fix now holds the gear strongly to the shaft as is required.

Operation of the drive mechanisms now took on a considerably different nature. The following motions were observed with no hysteresis correction entered into the computer. There was now no retrograde motion. Instead, there was no motion at all for about 3 seconds. This corresponds to 45 arc seconds of drive demand with no telescope motion. When stop action is called for, the tube now stops immediately when it should and subsequently does not move at all. This is both correct and necessary because it means there is no overshoot or drift. It does however require another 3 seconds for motion to take place in the opposite direction. This confirms the symmetry of the 3 seconds of hysteresis in the drive. This corresponds to 45 arcseconds of movement of the optical tube. This amount of delay is similar to that expected when the drive is operating to specifications stated in the operators manual.

One must conclude that when requesting reversal of declination motion, there is a total windup in the gears, worm and main gear of 45 arc seconds. This seems like a lot of windup in the gear train, but it is only a very modest set of plastic (with some metal) gears. The system as adjusted is now very tight mechanically, but still very smooth running. Since this wind up is symmetrical and consistent in amount, it can now be compensated for by the declination compensation of the computer. The declination compensation entered into the computer simply causes the drive motor, which is tightly controlled through its encoder, to windup the required amount in the desired direction so that mechanical lash is absorbed and the forces applied are just enough to start motion of the declination axis. In each of the final, adjusted drives, the wind up was about 45 arc seconds and could be easily compensated by setting the computer compensation.

What actually happens is that this correction, of so many arc seconds, is inserted into the declination drive command so that at the very start of the reversal of the drive command the motor jumps the amount required to take up the hysteresis in the gear reduction system.

Both drives, after many hours of remodeling and "tweaking" are now operating fairly well. They seem to be smooth and reverse with consistency. Apparently no amount of "tweaking" will make the coupling between the motor shaft upon which the encoder is mounted and the declination axis absolutely tight. This is to be expected with the very simple gear train used. Only expensive spring loaded gears as used in precision servomechanisms would be free of mechanical lash. Thus it is fortunate that a very clever computer fix for this problem has been provided. The mechanical hysteresis problem is probably extant in most drives of this type but is usually not amended.

The conclusion of this study and experiment is that the floating worm drive design while a bit unusual is probably necessary in a mass produced drive so as to account for production tolerances. And also that it is possible to adjust the drives to optimum condition by "tweaking" it on an individual basis. This may take several hours of careful mechanical reworking. In addition to tightening the looseness in the declination drive, it is useful to reduce forces due to unbalance and acceleration. The first is done by balancing the telescope tube carefully. The second can be reduced by reducing the slewing speed to less than 8, the default value. A slew rate of 2 is actually the same as the "find" rate which is still fast enough for most GoTo operations. Additionally the mechanism is not so noisy as to attract embarrassing comments from fellow viewers. As a compromise, a setting of 4 might be used. Immediately after the computer boots, I generally set the slew rate to 4.

I am pleased with the improvements I have effected by "tweaking" on the declination drive gearing. The actions are now very tight and very similar for N to S and S to N direction changes. The rebuilding of the drive that I have effected now brings both telescopes within normal tolerances of 2 to 4 seconds so they can be appropriately compensated using the declination lash computer setting. I would also note that "tweaking" has tightened up the drives mechanically a bit so it should be determined that the motor does not stall with any loads used. A stalled motor will heat up as will the driver circuits with possible dire consequences. Also note that the motor is least likely to stall when used with its full voltage ratings. Running motors run cool. Stalled motors get hot. That is why low commercial voltages or "brown out conditions" sometimes burn out motorized equipment.

I hope this study and analysis of the declination drive yields useful information to those who are having problems with it, want to understand better how it works or want to try to improve it or bring it into required specifications.

There are many details to be observed in the rebuilding of these drives. I am not recommending it be undertaken except by persons with considerable mechanical skill. Some electrical skill is also an asset. I will be happy to give more hints about doing the job to those who might be seriously interested in undertaking this project.

I accept no responsibility whatever for the results of any attempts to "tweak" the declination drive by experts or klutzes. If you klutz it up, it's your own fault.

Visit Doc G's website for more of his research
Clicking this link should open a new browser window.

Subject: LX200 Dec Drive System Function   

From: Doc G

This note is an attempt to clarify the basic operation of the LX200 servo control system used for both Dec and RA drives. I hope it will be of interest to LX200 users. Sincerely, Doc G

The issue of failure of Declination drives has appeared several times recently on this site. A question is, "Does the mechanical drive with its electronics only need to be replaced or does this unit and the main control board also have to be replaced?" Unfortunately, in the case of a pure runaway situation, I do not believe it is possible to tell. The reasons are given in the following and will I hope serve to clarify the operation of the drive motor and associated electronics for those that are not familiar with the operation of differential servomechanism controls.

I am not privy to the detailed schematic circuit for the LX200 controller and have had to construct its probable operation from an analysis of the drive and its electronics and the signals being sent to and from the control board. Differential servos are well understood so I hope this brief outline of their operation and potential will be of interest to LX200 owners.

The drive motor shaft has on it an encoder which sends two signals to the control board. When the motor shaft is turning, these signals are a pair of square waves which are 90 degrees out of phase. The out of phase signals are deciphered on the control board into two signals. One signal is a series of pulses which tells the control board how many slots on the encoder wheel are passing the detector and the other tells the computer which direction the wheel is turning. By keeping track of the number of pulses and the direction of the rotation of the motor shaft the computer knows where the telescope should be pointing with respect to where it was. The RA and Dec drives work in similar ways.

It must be understood that it is the position of motor shafts which are controlled. The computer simply tells the motor shafts how many turns to make to go to a new location. The gear trains and the main gear are not inside the control loop. Thus mechanical lash in these parts is not corrected by the servo system. The motor shaft is however very tightly controlled. The computer and encoders generate enough pulses to control the motor shaft to within one degree of the desired position. If there were no lash whatever in the transfer of this position information to the telescope tube, this one degree of motor shaft rotation would correspond to 0.3 arcseconds of tube motion. This is very high accuracy. In studying the motion of the gears and the pulse trains delivered to the control board, I believe that the motor shaft is in fact controlled to the accuracy claimed. This type of system is a very common control system which is well understood. It is usually called a differential encoder servo system.

The differential encoder system differs from what is called an absolute encoder because the differential system must be given a reference position from which it measures. The absolute encoder would relay the shaft position, or the telescope position, directly to the computer with an appropriate code wheel. The differential system requires simpler and much less costly encoders than the absolute system. It is never-the-less a good control system.

Unfortunately, there are several things that can go wrong with this type of control system that will make it "run away." Since there are two small lamps and two photo detectors which make up the encoder, failure of any one of them will send false information or no information to the control board. The control board power amplifier will however make the motor turn without end and think it is not moving at all or moving in the wrong direction. The encoder itself has a mask and a moving disk. The disk moves at one turn per 8 seconds for sidereal tracking rate and at about 20,000 RPM at full slew rate. If anything gets miss-aligned, there will be no signal and again may be a runaway situation. Both of these types of failure are on the drive mechanism itself. It is also possible that the minimal electronics on the drive circuit board will become defective, but that is not likely.

There are also several other ways the drive will go into runaway. One would be if the control board and its computer misinterpret the two pulse trains coming from the drive mechanism. Another is that the computer fails to count properly and sends the wrong drive signal to the motor. Another is if one of the power driver transistors should short out. This would send a full power drive signal to the motor causing loss of control.

There really in no way to tell from the runaway situation what has gone wrong. It is the nature of a feedback control system to do its best to satisfy the differential control commands. Any failure within this control loop will cause a similar symptom. It is also likely that a runaway situation will drive the RA or Dec into a mechanical limit and thus jam the motor with full power on it. This could cause motor burn out and/or power transistor burn out. This cascading effect is not uncommon in closed loop control systems.

If the cable connections are the problem, an open circuit in the encoder line will cause runaway and an open circuit in the motor line will cause an apparently dead drive.

I am sorry this does not help you fix any problems. It should however remind you that one failure can and often does lead to another. Replacing one part may not solve the problem and without detailed schematic circuits and appropriate electronic test equipment it is difficult to find the real problem.

Closed loop systems are most often fixed by replacement of sections of the system until it works again. (a la MIR, yes and automobile electronics and lots more)

Visit Doc G's website for more of his research.
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Subject: Dec Backlash Compensation South of the Equator --part 1 of 3  

From: Tim Long <Timlong-family.com> Date: Apr 2002

Some recent posts on various groups have rekindled my interest in the bug in Dec backlash compensation for southern hemisphere users. The claim is that backlash compensation works in reverse for observers in the south, rendering it completely useless and often mistaken for retrograde motion due to poor declination bearings.

I discovered that the thing that triggers the problem is to enter negative latitude in the site information. The setting of the N/S switch does not affect the problem (maybe it should).

Another trick people use is to do a deliberately less than perfect polar alignment, so that the target drifts in a constant direction in declination. Then in the camera control software, guiding is only allowed in one direction. That way, the issue of backlash is avoided because the Dec motor is never asked to reverse. I haven't tried it myself, but I think the trick is to get the misalignment large enough that the object drifts sufficiently but not so large that field rotation (and keeping up with the target!) becomes a problem.

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Subject: Dec Backlash Compensation South of the Equator --part 2   

From: Doc G

The scope works well in the southern hemisphere. In fact if you have a POLAR alignment that is excellent, you almost never need declination correction at all. Thus you can set the backlash to zero. In fact some imagers disconnect the declination drive completely during guiding since the declination motor vibrations agitate the OTA.

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Subject: Dec Backlash Compensation South of the Equator --part 3 of 3  

From: Paul Luckas, Date: June 2002

I've had confirmation from Meade today that the declination backlash compensation problem for negative latitudes (i.e., southern hemisphere) is software/firmware related. For those unfamiliar with the saga, declination backlash compensation doesn't work for negative latitudes on the original/classic LX200 series. In effect it works in reverse -- the star corrects in the wrong direction. Given the minor nature of the problem and the expense required to rectify, it is unlikely that Meade will offer a fix --and understandably so. In my case, fully unattended robotic imaging works just as well without declination backlash compensation. For casual observing, one can generally live with a 3 second delay in the reduction gear box when changing direction in declination at guide speed. For those who helped verify the issue -- thanks. It is nice to have closure on something that was for a long time confused with retrograde motion and other mechanical issues.

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