Elimination of Vibrations

       

Subject: Elimination of Hi-Frequency Vibrations 

From: Ed Stewart <stargazerskymtn.com>

The all metal construction of the LX & its tripod makes it prone to high frequency vibrations from slewing, DC humm from the drive motors, and accidental physical contact from other metallic objects such as eyepieces, cameras, etc. I've found a way to virtually eliminate these vibrations while attempting to make the scope easier to use.

First, I wanted an accessory shelf, such as the BB200 advertised in the mags. for $99, that installs between the OTA base and the tripod head. I noticed that it was constructed from 1/4" or 6mm thick aluminum. Having used 6mm Baltic Birch plywood on a number of telescope construction projects, I felt the plywood was a better material to use since wood is very effective in vibration damping, and I had the tools to work with wood. After drilling the various eyepiece holes, installing register pins to guide the OTA onto the tripod screw, and making a angled rest for the control paddle w/joystick, I noticed an immediate reduction the hi-freq vibrations, but not completely. To see this accessory shelf, return to my homepage.

BTW, I've been asked why I suggest using Baltic Birch plywood in my projects. Because it is made of better quality inter plys of 1mm, there are more plys per inch thickness and they are stronger. So much so that it has nearly the qualities of aluminum. There are similar plywood on the market. I spotted one at Home Depot, and The Woodworkers' Store, 21801 Industrial Blvd., Rogers, MN 55374-9514, carries a marine grade that is comparable in 0.25"x12"x16" for $2.65 and 24"x32" for $9.95.

Second, when lifting the tripod by putting my hand under the tripod head between two of the legs caused the rather sharp edge of the head casting to cut into my palm. So I split 3 lengths of 1/4" inside diameter automotive fuel line and glued them (w/Goop) onto the casting's edge between each leg. This makes a much larger and softer edge to bear against the palm. While working in this area I noticed that the spreader arms being metal and in direct contact with the tripod legs would transmit vibrations between the OTA and the legs. So I split three 1 1/4" (+ or -) lengths of the same tubing and Gooped them to the spreader arms where they make contact with the legs. This broke the metallic link between the legs and the OTA. If you decide to try this, I strongly suggest using *Goop*, because it cures in a day to an extremely tough but slightly elastic bond. Be sure to get a variety that states it is "UV resistant", or it will turn brittle when exposed to sunlight.

I can no longer detect hi-freq vibrations even at 250x. And low-freq vibrations that come from the wind or from touching the scope are greatly diminished also.

Subject: Vibrations, the AO-7 and More 

From: R. Dalby & Doc G, Date: May, 1999

I saw a post on the SBIGuser site which was so well said and so very appropriate to users of fork mounted telescopes, others as well, that I wanted it to be shared with the MAPUG group. The post came from Mr. R. Dalby who is one of a group of very fine CCD imagers in the UK. His excellent work can be found at <http://www.bizvis.demon.co.uk/hfo//> Note: should open a new browser window over this one.

With his permission I have quoted most of his post with just a bit of editing and added, at the end, a few of my own experiences. I thank him for sharing these cogent thoughts with all.

Quote from Mr. Dalby:
I'd like to offer a few simple thoughts and observations concerning high resolution imaging with the AO-7. (As applied to an LX200. ed.) I would certainly hold that nine tenths of the damage done to starlight occurs in the last fifty feet from the chip! It's ironic, and not a little annoying, that the light travels trillions upon trillions of miles only to be spoilt in the last few feet of it's awesome trip to our detectors!
My own experiments employing real-time CCD cameras on my 12" SCT positively frighten me. The sheer amount of movement caused by wind, vibrations from the local environment, vibrations from the imaging system (motors under load, general drive perturbations cable settlement etc.) and last but by no means least the OBSERVER, confirm that the greater part of the correctable causes of stellar wander stem from the scope and the local environment rather than remote air turbulence.
Let's be clear here: I'm talking about MY system in particular. I am not passing any kind of judgment about everyone else's. So if your wonderful, rare, expensive, etc. imaging system doesn't have any of these hang-ups don't feel you absolutely have to tell me about it - take it as read. I'm suggesting that it might be useful to assume that yours may be enough like mine, in some areas, to make looking at some of these issues worthwhile - that's all. (Mr. Dalby is discussing the 12" LX200. ed.)

I think that there is much variation in the range of seeing conditions
that pertain to one site vs. another. I've noted a wide range of seeing conditions from my own site in the same night! I would recommend all readers of this list try attaching a real-time CCD camera to there scope - if they have the chance - it's a mighty sobering exercise.(e.g. a sensitive video camera. Ed.)

People who look through an eyepiece stamp their foot and think they have got a well isolated system are in for a big shock. I'm now of the opinion that most talk one sees of "isolated" systems is, more or less, complete twaddle. You need a frame of reference to do this kind of test and a TV monitor is ideal. Focus up on a bright star and then get someone to move carefully around in the observatory. You'll see every footstep! At f20 I can easily see the "earthquake" caused by someone's foot falls as they walk away at up to 50 ft. As for moving around in the observatory - it's an absolute disaster - don't do it! The oscillations can be easily 3 to 10 arc-seconds and can be detectable for as long as 2 to 5 seconds. My pier is a 12" diameter and concrete: about 6'6" is in the ground. It's fully isolated from the concrete pan, and the floor is wooden and 2 foot up from the pan. I sit in a warm room that has separate foundations about 12 foot from the scope. And yet if I jerk my chair forward to get nearer the desk the pier "rings like a bell" and the TV monitor shows a 1.5 to 2 arc-seconds "quake". Isolating piers is a typical Archimedean conundrum - if you stick it in the ground regardless of what you do to separate it from local floors vibration will still get on to it. Because something has to support the floors unless you have anti-gravity timber - and this means any vibrations goes from the floor into the supports into the ground and then through the ground onto the pier.

Now some folk are thinking "he doesn't mean me - I spent a lot of time thinking about this and went to a lot of effort to isolate my system and mine is really good." I'm suggesting that this is a very common and very unsafe and assumption - and until you actually prove that your system is indeed well isolated (with a real-time CCD camera working at a high focal ratio, say f20) it is procedurally safer to adopt a skeptical position and implement at least some of the recommendations at the foot of this post.

It's important to grasp from the outset that with high resolution work (<2 arc sec) the scope/imaging system doubles as a very sensitive seismometer. At f20 it even works like a microphone -- if you talk loud near the scope it is recorded by the star motion! To counter suspected local bad air problems I am currently installing two large extractor fans in the hope that the more air I push out of the dome the less will stream out through the slot overhead. I also plan to try a modification to my 12" LX200 by attaching fans to the tube to replace the chaotic thermal turbulence within the tube with a steady positive air stream. Of course, these fans may cause problems of their own and I will be careful to check out the impact of their use with the real-time camera test.

I still remain skeptically disposed to assertions that the AO-7 is incapable of correcting for seeing dependent stellar wander. I see nothing in the AO-7's design that actively prevents the favorable corrective impact on shift caused by some air turbulence and I am surprised that some hold the negative view. However I generally accept the weaker implied contention that the role of the device in correcting seeing is of a much narrower extent than is often supposed.

Conclusions and recommended action: (in no particular order)

1. Do the real-time CCD test. Almost any security type camera can be used for this exercise. Use a Barlow and focus up on a bright star - use post-it notes or masking tape to make a smaller on-screen frame of reference around the star - and brace yourself for a shock!
2. Examine closely all local (up to 50 ft) sources of vibration. Trees, fences, roads, paths, (washing machines!) etc.
3. Implement an ABSOLUTE restriction of movement in the immediate vicinity of the scope during an imaging episode. E.g. obviously you should avoid dome rotations during exposures. Take steps to "police" people movements up 50 ft from the scope.
4. Remove or reduce the size of dew caps. They act like sails catching any air currents around the scope.
5. Tie all cables to mount/scope superstructure. That is, avoid allowing the cables to simply trail away from the camera - the cables will settle from time to time and they act like antennas picking up vibrations and transmitting them to the scope.
6. If you have an enclosed observatory (dome) take steps to stabilize internal air currents and reduce the total amount of warm air moving through the overhead slot (see above).
7. Look carefully for signs of, typically oscillatory, vibration emanating from the moment that the drive motors (RA) are under load. Increased inertia from excessive payload weight and poor balancing can be a source of problems here.
8. Scope and camera fans. With a real-time CCD camera fitted to the main optical system and addressing a bright star but with the ST-7,8 still fitted to the scope; experiment with switching the fan on and off. Surprisingly it is possible for fan induced vibration to be relatively slow and low frequency, taking a few seconds to build up, so be patient. Check fan blades for the build up of debris as this may cause the fan to be out of balance.
9. There is a strong case for fitting fans to the telescope tube to stabilize air within the OTA. I'm looking carefully at this modification for my 12" LX200 at the moment.

As has been discussed earlier, the AO device is very effective at countering much of the vibration that I have enumerated. However, the AO-7 is reactive in character, therefore the less work it has to do the better it will be for the image and the better position you will be in the exploit those all to few really good nights.

A few additional comments from Doc G:
The above post was to point out that there are many vibrations of the equipment which can be at lease partially corrected using the AO-7 stabilizer. I want to emphasize that these vibrations and many aerial defects while very small, function at the level where they just take the edge off of the sharpness attainable with the best optics.

I have done some planetary viewing using a video camera and because of the high magnification necessary, found all of the vibration sources described above. There are, in addition, aerial disturbances, many of which take place in the last few hundred feet before the telescope that have to be considered in more detail. In particular, when viewing at relatively low declinations, such as at planets, I have observed that there should be no people in the path near and especially directly below your line of sight.

Several times I noticed a very sudden deterioration of the "seeing" while observing Mars and other planets only to find that a small group of other observers had gathered directly under my line of sight. The heat from their presence rose into my optical path and totally destroyed the image. Mars was jumping about several diameters much less a few arc seconds. Such gatherings have to avoided or shooed away!

Finally, the principle point of this discussion is to remind everyone trying for high precision pointing and imaging that everything mechanical is important and anything that disturbs your near air path must be eliminated or optimized as well. Even when your focus is perfect as possible, if the stars are taking tiny random walks in the image plane, the exposure with integrate over the moving image and it will appear out of focus or bloated.
Sincerely -- Doc G

Subject: LX200 (classic) Jittery Tracking Causing Image Vibrations 

From: Doc G, Date: Feb 2001

Larry Owens wrote:
While observing the Moon with my LX200 10" I noticed what appears to be a slight jitter or stutter in tracking resulting in image vibrations.

This occurred while in Alt-az mode. The scope was purchased last fall. I first attributed it to atmospheric conditions but as I continued to pay attention to this, I noticed that it seemed to correspond to the tiny ticks of the RA and/or DEC motors. Observing again this past Saturday night I noticed the same thing, conditions much better this time, but still getting a bit if jittery tracking or image vibrations on occasion.

This is not something that I can noticed while observing deep-sky objects, and it doesn't occur all of the time. It is a bit annoying while performing lunar and planetary observations though and it may affect deep sky astrophotography, which I have not done with the LX200 yet.

Again, I'm very acquainted with the observational affects of the atmosphere and do not feel that this is the problem. My theory at this point is possibly a bit of play between the worm and worm wheels, or something along this line. Anyone else experience this or have an explination or solution?<< End quote

This is a problem that has come up many times. It is more of a problem when in Alt/Azm mode. The reason is that in this mode, both the RA and Dec motors are pulsing. These pulses tend to cause oscillations in the mount for some peculiar cases where the loading of the scope and the frequency of the pulsations is just right. (wrong in this case) However, it is a real problem with some setups that accidentally resonate at the wrong frequencies.

This is a mechanical resonance problem which you might be able to modify in several ways:

1. Increase or decrease the mass of the OTA by adding or taking off some attachments.
2. Change the balance of the scope. Forcing the scope to load the drives differently will change the torque on the drives.
3. Hang a piece of rubber covered chain on the OTA. This has a damping effect on the mount that might reduce the vibrations you are seeing.

Subject: Poor Seeing/Image Vibrations caused by Tracking Motors (classic) --part 1 of 5 

From: Doc G, Date: Aug 2001

William R Wood wrote:
> Thanks for this info, even thought it sounds like bad news. I did check
> your site first and saw a post about vibration and I did try hanging weights
> on the scope as suggested to dampen the vibration, but with no success.

This is one of those finicky problems that can come and go. I think all of the LX scopes have the problem but many users are not bothered because they do not do a lot of higher magnification viewing.

There is no clear solution that I know of. The rubber covered chain has been known to help and some have found hanging a weight on a bungee cord helps. This is such an erratic problem that I know of no sure cure.

> When you say this is a well known problem and there is no solution except to
> turn the Dec motor off are you also saying that this problem affects
> everybody or just an unlucky few. If only a few are afflicted it may be
> worth making a warranty claim for a replacement. What would you suggest
> that people with new scopes still under warranty do.

It is not likely that a new scope will solve this problem. I doubt that Meade will even recognize that this is a problem. I think they all do it to some extent or the other. The best solution is to go to equatorial mounting and turn off the Dec motor. You polar alignment has to be very good but not perfect for reasonable length exposures. For viewing polar mounting should help a lot. The RA motor does the same thing, but it is much more isolated from the OTA and thus does not cause serious viewing and imaging problems. I have heard from a small number of owners that they can see the RA motor running as well. This effect is not well documented.

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Subject: Poor Seeing/Image Vibrations caused by Tracking Motors --part 2  

From: Doc G

This sounds like some good advice. Since the offending scope is under warranty this can be done without great cost.

Gordon Lupien wrote:
> Within the first week of buying my 12" LX200, I noticed a grinding clicking
> sound in the DEC motor that was periodic (like, every couple minutes while
> the scope was tracking). I experimented a bit and found that there was a
> click/grind whenever I slewed too. I had heard that the scopes were loud,
> so I thought that's just life. Then I noticed one night that when the
> click/grind was happening, I SAW it as a sort of "hitch" where the stars
> jumped a bit. I thought that was wrong, so I called Meade tech support and
> had the fellow listen to the DEC motor over the phone. He said it wasn't
> supposed to click/grind and promptly sent me an electronics replacement kit.
>
> The scope no longer click/grinds and is actually quite quiet. The motors
> now sound like a working gear box should, basically a consistent whirr with
> no added clicks or grinds. The images are rock-solid now. (I also greased
> the heck out of the worm assemblies too. Half of the large gear was not
> even lubed!)
>
> Anyways, if it's CLICKING or GRINDING, it's probably defective or broken.
> They make noise, but they don't click or grind. Either that or mine is very
> special...

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Subject: Poor Seeing/Image Vibrations caused by Tracking Motors --part 3  

From: Peter Erdman <erdmanpedb.erau.edu>

This is a generic problem for any driven mechanical system. Under some conditions the amplitude of an oscillation will grow when the driving frequency approaches the natural frequency of the system (resonance). You are seeing it in Alt-Az, but it has also been mentioned by people using the LX200 with the equatorial wedge. It has also been mentioned as a severe problem by people using AP1200s. It is certainly not Meade specific.

It is not likely that the system's natural frequency is well defined (i.e. low Q), so changing the driving frequency slightly won't have much effect, as you have noticed. You need to introduce damping or increase the energy necessary to drive the oscillation. Both were suggested below. It is easy to see if damping is effective by placing a finger at a suitable location on the OTA. Suitable is of course defined as something that works.

Moving attachments (cameras, finder scopes, etc.) around will change the moment of inertia of the system and its natural frequency. You tend to be rather limited here in what you can do. Too much weight can be just as bad as too little, so that heavy add-on scope may aggravate the problem. Putting a TV 102 on a 12" did it for me. Too much mass coupled with limited stiffness in the system. Bad combination. Back to the Pronto.

I have found that changing the balance has the largest effect. The first time I noticed this problem was immediately after I had carefully balanced the system. The stars were all little vertical lines (Alt-Az mount). The slightest vibration would send it into oscillation, but the slightest finger pressure would stop it. Obviously not much energy here, but way too much amplitude.

So I did what was suggested here ages ago, loaded the driving gear so that the worm gear was forced to "lift" the unbalanced weight--which is to say it had to positively drive against gravity. The worm gear could also be loaded by the worm wheel "falling" against it, but that seems to be a mode that induces chatter in the gear drive (probably due to varying stiction in the greased gears). This solution was pointed out by people running in the equatorial mode, who deliberately put weights on their forks to slightly imbalance the system and reduce RA vibration.

In my case (and probably yours) the driving worm causing the vibration was the DEC drive, and thus the direction of the preferred imbalance will depend on where you point in the sky. After all, in the East the scope must be rising in elevation angle with time, but when pointed to the west its elevation angle is falling with time. To the South the DEC drive isn't doing much of anything. You can try the experiment of pointing to different directions and observing how the vibrations might change.

I don't think that you are fundamentally doomed to endure this oscillation. As I mentioned in a previous message, I recently had the best viewing of Saturn that I've ever had (~400X, Alt-Az). No vibration noticeable in the mount or the sky. One of those wonderful moments that help justify the time and money expended.

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Subject: Poor Seeing/Image Vibrations caused by Tracking Motors --part 4  

From: Bill Wood <wrwoodqwest.net>

I posted recently about image vibration caused by the tracking motors. No obvious solution emerged but Doc G indicated that the Dec motor was likely the problem so I removed and inspected it but I saw nothing obvious wrong. When I ran the motor while it was off the scope it ran fairly smoothly with no odd noises. As mounted on the scope by the factory, it had an odd clicking or ticking noise while the vibration problem was occurring during tracking. The gearbox under the Meade rubber tape has a rather cheesy sound on or off the scope, probably due to the plastic gears. I then removed the outer clutch plate and the big Dec gear. I cleaned all the Meade grease from the worm gear and the big Dec gear. No chips, grit or other debris were present. The worm and big gear are free from obvious defects. A dial indicator on the worm gear shaft shows runout of only .0005 at the motor end and near zero runout at the far end. The motor pivot was somewhat stiff at first. I flexed it back and forth over its full range of motion a few times and it loosened up considerably. It now pivots freely on the pivot balls but the pivot is tight enough that there is no slop. I did not change the factory adjustment of the pivot tension.

The big gear has side to side runout of .020 over a full 360 rotation which seems excessive. This wobble is caused in part by the inner Dec clutch plate which has .010 runout itself. The aluminum Dec gear contributes the balance of the runout. I reoriented the outer clutch plate by turning it 180 deg. but that had no effect on total runout. The up/down or concentric runout measured at the tops of the teeth of the big gear is under .005 over the entire circumference. From watching the gears run I don't think the Dec gear wobble causes any material problem and doubt it is responsible for image vibration.

The Dec motor mounts to the fork housing via two elongated holes. You can slide the motor left/right along the elongated holes and up/down slightly. I fiddled with this adjustment with no particular change visible when running the motor. I put the motor back with the bolts centered in the elongated slots. Meade had the motor mounted with the bolts hard up against the right side of the slots.

What does make an immediate change is to press down slightly on the motor housing while it is running so as to slightly reduce the upward pressure of the worm gear on the big gear. All of the funny whining noise disappears and the motor whirrs much like a decent motor is supposed to sound. A weaker spring would not produce the same result because a slight disengagement of the gears is necessary before the sound changes from whining to relatively quiet and smooth. This slight disengagement is obviously not good because now there is slight gear slop and backlash.

As mounted by the factory the worm gear is not centered side to side under the width of the big gear; it is in too far. I shimmed the motor out with a rubber washer between the motor housing and the fork housing made from the rubber tape Meade installs over the gear box. This centers the gear a little better and may provide some vibration isolation. I replaced the Meade rubber tape over the gearbox with duct tape.

I regreased the clutch parts with red general purpose light bearing grease. I regreased the worm gear and big Dec gear with the very high quality black moly grease that I use on my semi-auto target pistols. I also added some clear light fishing reel grease to the plastic gears in the gearbox. The plastic gears come with a small amount of tan grease but my clear grease is very sticky and was made for high quality bait casting reels that also use plastic gears and it does not fling off like the Meade grease does. A drop of very high quality synthetic semi-auto gun oil on the Dec knob shaft completed lubrication.

The Dec motor sounded smoother after the relube but it still whines and ticks. The slightest downward pressure on the motor still makes it run much smoother and quieter but that does reduce the gear engagement and thus creates gear lash.

For reasons I cannot specifically put my finger on, this operation cured the image vibration. Out at 3:30am today and Saturn and Jupiter were solid as a rock over the 2 hours I observed them. Seeing conditions were very good and there was no wind. I could not detect any difference between the image while the scope was tracking versus the image with the tracking motors turned off by holding the E key in Guide mode. The Dec motor's sound while tracking is definitely different, there is a ticking sound but the ticking is now regular, before it was irregular.

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Subject: Poor Seeing/Image Vibrations caused by Tracking Motors --part 5 of 5 

From: Doc G

My mental review of the posts on this issue for the past several years (with allowance for senior moments) is that the situation is worse in Alt/Azm than in Equatorial mode. The reason being that both motors run in Alt/Azm while only the RA motor runs constantly in Equatorial mode.

Since the RA motor is physically remote from the OTA and the mass of the fork mount tends to give additional isolation the RA motor has a small vibrational effect on the OTA. With excellent equatorial mounting, the Dec motor can be turned off completely thus reducing the overall vibration.

This vibration is caused by hunting of the closed loop control system that drives the motors. At this time I do not know of a solution, However, it is possible that some form of mechanical damping might be introduced in the motor shaft itself or in the electrical part of the control loop. I have not experimented with this problem yet.

Subject: Image Vibrations Solved (Jittery/Jerky Tracking)  

From: Bill Wood <wrwoodqwest.net> Date: Oct 2001

Some time back I wrote about image vibration (jittery tracking) caused by the tracking motors. In the process of resolving that issue I contacted Meade and was provided (under warranty since my scope was only 4 months old) with a service kit consisting of a new mother board and new RA and Dec motors. Meade believed the vibration was caused by defective electronics.

The image vibration problem was noticeable only on Saturn and Jupiter and looked exactly like poor seeing conditions. I detected the vibration by turning off the tracking motors which stopped the vibration. The vibration returned as soon as the motors were turned back on. As it turned out the vibration was cured by simply removing the original Dec motor, the Dec clutch and big gear then cleaning and regreasing everything and reinstalling it carefully. The Dec motor is adjustable in relation to the big gear to a certain extent because the holes in the motor mount have some play around the mounting bolts. You can vary how close the worm gear is to the big gear and you can vary the angle somewhat by tilting the motor up or down. In addition the Dec motor can be shifted right or left. From the factory my Dec motor was shifted right and tilted down. When I replaced it I shifted the Dec motor left to clear the big gear as much as possible and made it level and straight in relation to the big gear. The motor noise was greatly reduced, the worm turned much smoother and the big gear no longer hit the soft rubber tape over the rear of the Dec motor. The image vibration disappeared and my Dec operation was substantially quieter and smoother overall.

Additional note: When the motors are out and in your hand carefully flex the hinge that the motor mount pivots on and feel for stiffness or play. This is the hinge that the little spring pushes open and thus holds the worm gear in contact with the big RA and Dec gears. There should be absolutely no play and absolutely no binding or stiffness. My Dec motor from the factory was way too stiff; the RA motor was a little stiff. If you moved the hinge on the Dec motor it stayed where you moved it. The spring could not overcome the binding. I did not loosen the adjustment screw to fix this. I simply flexed the hinge repeatedly until the binding was eliminated. I believe there was a little burr or something in the way and after maybe 20 flexes the motion was smooth and free but there was still no play. Be careful of the wires while flexing the hinge - you don't want to break them off by accident. When I reinstalled the Dec motor and pushed down on the gear housing the motor now snapped back up into firm contact with the big gear. Before I removed it and did the hinge flexing, the motor would not snap back and the worm gear would stay partly disengaged from the big gear. The spring tension was not strong enough to overcome the binding. Make sure both RA and Dec motors snap back into full gear mesh after they are installed. You may have to adjust the tension screw which has some type of Locktite on it if your motors are too tight or too loose. MAPUG-Astronomy Topical Archives describes and has excellent photos of how to do this procedure too.

Given the extreme improvement in Dec performance I decided to service the RA system as well. I had a tight spot in RA which caused the pitch of the motor to increase noticeably over about a 45 degree part of the rotation because the motor had to work harder to turn the scope over that area. I completely disassembled the RA assembly and gears. It is very easy and took about 15 min. following MAPUG-Astronomy.net instructions and photos. I did not remove the big RA bearing which was stuck on the RA base. There was no need to remove it since it was clean, straight and turned very smoothly. Then I cleaned the parts and regreased them. Reassembly is key. There was nothing major wrong from the factory installation, no dirt or metal chips, just lack of extreme care in aligning the various parts. First make sure you have grease on the parts evenly. Assemble and disassemble everything 5 or 6 times for practice til you get the feel for how the parts should work together. On final assembly carefully center the parts of the main gear assembly and slowly tighten bolts in a crisscross pattern increasing the torque in steps. When I had the parts back together there was absolutely no tight spot. The entire assembly was now round and free of any binding. Reinstalled the RA motor so that it was shifted left for maximum clearance of the motor housing and the big gear and straight and level. RA motor noise was now much quieter and consistent all around a 360 deg. rotation.

Then the service kit arrived. Even though all of my problems were already solved I decided to install the kit anyway. I replaced the main board the original RA and Dec motor; the job took about 15 minutes since by then I was very familiar with the inside of the LX200. I was very careful to see that both motors were properly aligned to their respective gears. The service kit worked fine on my workbench. On star testing I noted that the RA motor behaved oddly. There was retro motion when I pressed the E or W direction keys and the star image would jump up before it started to move sideways. I called Meade and told them the new RA motor was wacky and asked if I could replace it with the old one which had been working perfectly. I felt I had to ask since the service kit was called a matched set consisting of specific motors tested with a specific main board. My request was approved after some consultation and I reinstalled the old RA motor which worked perfectly. All retro motion and jumping were gone.

The good news is that my pointing accuracy was vastly improved by the service kit or the Dec/RA service or some combination thereof. Before I installed the service kit and did the Dec/RA cleaning and regreasing, I was getting pretty consistent 2 arc minute pointing accuracy. After the service kit, etc. my accuracy more than doubled to under 1 arc minute. This is without HPP. Some nights I now hit targets within 30 arc seconds or less. It is amazing. Maybe some electronic sets are more accurate than others or maybe the better adjusted RA and Dec drives allow the motors to work better or maybe a little of both. I also use BestPairII and a reticle EP and make sure there is no backlash as I center the alignment stars. I have also made the OTA orthogonal to the Dec axis.

Bottom line is Meade was very helpful and willing to assist me and the scope certainly performs beyond the accuracy claims in Meade's adds which at one time I questioned. Thus I take back anything bad I ever said about Meade and compliment them on making a fine product at a fair price. I do still believe that Meade should sell a complete shop manual with good photos, drawings and instructions.

Subject: Vibration Sensor Recommendation

From: Doc G, Date: Dec 2001

As Mapuggers know, I have written extensively about LX200 mount vibrations, oscillations and wobulations for several years. I have studied these problems because they are of interest to me professionally, but also because they are a significant problem in all aspects of using the LX200 for imaging.

For many years I have used a pricey geophone, a multichannel oscilloscope and a spectrum analyzer to measure LX mount vibrations. These instruments give me a very precise, detailed evaluation of the vibrations. Unfortunately the instrumentation is many time the cost of the LX200 telescope.

I have run into and had a chance to test a gadget that measures and gives an indication of the vibrations of the telescope that is a very useful instrument. It is the Scope Sentry from Eztelescope. This unit is a well built box with a nice saddle mount that can be fastened to any OTA. It has a series of lights on the front that indicate the level of motion of the OTA. I have had a great time playing with it. It will tell if the telescope is shaking from whatever source, ground disturbances, stomping around the pier, wind or whatever.

This is not a tool for detailed vibration or spectral analysis of the vibrations in the OTA. But it is a very useful tool to judge when wind vibrations are too great to allow for good imaging. It is excellent to determine whether movement about the pier or in the observatory are causing OTA motion. It can serve to detect when wind is causing vibrations of the OTA. And, it can be used to tell if changes made in the mount, damping and other mechanical changes are helping or hurting the sensitivity of the mechanical structure against vibrations.

I am so please with this gadget, toy, that I thought it worth commenting to MAPUG-Astronomy and the general astronomy community about it.

You can see more about this neat toy at: <http://www.eztelescope.com/>

Subject: Dampening Vibrations in the LX200 Mount

From: Doc G, Date: May 2002

Assaf B. wrote:
Last night I tied a piece of steel chain to our 16" LX200 OTA and for the first time in the last few months the amount of vibrations was much lower. Until now the scope was vibrating in both axes and now after adding the chain on top of the OTA (usually where the Meade counterweight set is) the vibration in the DEC is almost gone.

With such logic I tried adding more pieces of chain to the fork arm and on different places on the OTA but without 100% success.

Can anyone recommend how can I choose the optimal place for the chain and how can I pick the size of the steel pieces of the chain (Im currently using a chain about 60 cm long and a diameter of 6 mm (which makes each piece about 1.5cm's width). ---End of quote.

You are using a well known technique for reducing vibrations in mechanical systems. What is happening is that the damping caused by tiny motions of the chain links is absorbing some of the mechanical motion of the OTA. This is a good and valid technique. While there is no easy way to determine exactly where the chain should be placed. It would be best to place it where the OTA moves the most. That would be at the corrector or at the very back of the OTA. The chain will help most with oscillation in the vertical plane, but some in any plane.

It is possible to improve the chain by using chain with rubber or plastic coating. This improves the damping action of the chain. It is also possible to use a combination of bungee cords and weights to accomplish the same thing. This is a very experimental thing. Some have reported good, effective results and other have reported no such success.

But in any case, you can experiment with these dampers which are simply called "passive vibration dampers" and you might have good results.

Just a reminder. Adding mass does not improve damping. To get damping you need a rubbery or gummy substance that is exercised by the motion you are trying to damp.