This information is for those interested in telescope motion, vibration and oscillation. This information is about the static deflection characteristics of a Meade 10 inch LX200 mounted on a Giant Field Tripod with and without a SuperWedge. This information has be checked for accuracy and some re-interpretation has been included as the result of added experience since March of this year.
The purposes of providing this data are two fold. One is to show that for this telescope and mount there are specific deflection characteristics related to the position of the applied force and the various bearings which make up the mount. The second is to provide information about where the weakest points are so one can determine where it pays to spend effort in an attempt to improve the stability of the mount and thus reduce vibration.
This information is for only one instrument. A different sample of the telescope may provide somewhat different results. On the other hand, these results are entirely consistent with measurements which I made some time ago on a 12 inch LX200 mounted on a very strong permanent pier.
This data is for static deflection of the main tube. However, there is good correlation between the static deflection and the sensitivity of the main tube to vibrations caused by wind, motor drives, bumping, irregular drive forces and the like. So it has considerable value in judging stability for imaging. I strongly believe that knowing these values really helps to both understand the telescope and its mechanical mounting and reinforces the ability to improve it. I have been seriously trying to get pointing stability to be as good as the best seeing conditions. This is in the area of 1 arc second.
The 10 inch telescope was set up in my basement laboratory with a series of weights, strings and pulleys arranged so that the telescope could be pulled with a known force in any direction. The tube was focused on a target calibrated in arc seconds of deflection. The setup is very simple so that anyone interested can easily duplicate it to measure their own telescope.
Here are the results for the telescope mounted directly on the giant field tripod using only the center bolt, very firmly tightened. The legs were not extended and the tripod sitting on a cement floor. The tube was pointed horizontally and the fork set up with its long dimension parallel to the front of the control panel. The force was applied by a 1 kilogram weight applied directly or via the string and pulley arrangements. All deflections given are arc seconds per kilogram.
Forces are applied as follows and deflections obtained given. The units are arc seconds per kilogram.
Force applied to the tripod head back and forth and side to side gave a resulting motion of 2 arc-sec.
Force applied to the fork forward and back: motion 20 arc-sec.
Force applied to the fork side to side: motion 15 arc-sec.
Force applied to the fork up and down: motion 1 arc-sec.
Force applied to the tube:
up and down at the eyepiece: motion 60 arc-sec.
up and down at the corrector plate: motion 40 arc-sec.
up and down at the top center: motion 2 arc-sec.
left and right at the eyepiece: motion 50 arc-sec.
left and right at the corrector plate: motion 30 arc-sec.
left and right at the top center: motion 2 arc-sec.
Conclusions: The tripod is very solid and should not be a concern with this mode of mounting. About half of the deflection seems to be at the fork bearing and about half at the declination bearing. This data is good news and bad news. The good news is that the tripod is amazingly stable when placed on a solid concrete floor. Adding weight to the tripod or other braces cannot hurt but will not do much good either. More spongy surfaces might be made more stable by adding weight because the tripod legs are forced into the surface more.
The bad news is that about half of the motion is at each bearing. The left and right and the up and down motions are difficult if not impossible to separate. However, the deflections for the fork bearing are about half the total for both fork and declination bearings. Thus about half is in each and that means that for significant improvements to be effected both have to be improved.
For polar mounting using the Superwedge, the measurements are as follows:
Force applied to the tripod head back and forth and side to side. Resulting motion 2 arc-sec.
Force applied to the fork forward and back: motion 10 arc-sec.
Force applied to the fork side to side: motion 4 arc-sec.
Force applied to the fork up and down: motion 10 arc-sec.
Force applied to the tube:
up and down at the eyepiece: motion 60 arc-sec.
up and down at the corrector plate: motion 30 arc-sec.
up and down at the top center: motion 15 arc-sec.
left and right at the eyepiece: motion 5 arc-sec.
left and right at the corrector plate: motion 4 arc-sec.
left and right at the top center: motion 4 arc-sec.
One major conclusion has to be that the telescope behaves quite differently
when polar as compared to Alt/Azm mounted.
The tripod and the Superwedge structure are very solid. The tripod and wedge act very much like a solid unit. Flexing of the Superwedge is insignificant compared to that in other parts of the fork and bearing structure. The side to side stiffness of the fork is much improved in the polar mount. This fact is verified by the similar motion of the tube to up and down motion but by the greatly improved stiffness of the tube to side to side motion. This does not necessarily mean that the polar mount will, in practice, be more stable however as described below.
This result is entirely in agreement with bearing theory. That is, the bearing stiffens greatly when it is loaded. The great loading upon the fork bearings when the telescope is in a polar mode is clear and the resulting stiffening in the lateral motion is clear as well. It is for reasons of stability or stiffness that bearings are often "pre-loaded." Thus contrary to expectations for stiffness of the telescope tube, bearings and mounting system, the entire machine seems to be quite stable in the polar mount mode. At the same time, the great dominance of one direction of instability, up and down motion at the eyepiece, is very bad news. It means that the stability of the telescope is dominated by this motion in both modes of mounting and that it is the largest component.
The statements in the preceding paragraph should not be interpreted as saying that the telescope, in actual use, actually wobbles less in the polar as compared to the Alt/Azm mode. In fact, forces which tend to make the telescope move in either mode probably tend to be caused gravitationally. This means that the Alt/Azm mounting seems much more stable since the telescope mount is very stiff in the direction of the gravitational vector.
The worst news is that if the springiness is mainly in the bearings, as it certainly seems to be, there is almost nothing that can be done about it. Only a major redesign of the mount would enable improvement. Reducing wobbles must then be accomplished by reducing any irregularities in the drive motion and any external forces such as wind, stomping around the pier or tripod supporting surface and definitely not bumping or even touching the telescope while imaging.
I have checked out bearing specifications and standards in various mechanical design manuals and these results are quite good for the bearings used and the way they are mounted. The main problem with the design is that the bearings are very close together so that the forces generated by the torque in the polar mode are very large. For two bearings to provide excellent stability of a shaft, they must be separated more. I have come to believe that the bearings used by Meade are quite good and in fact probably as good as can be expected within the cost parameters of the mechanical as provided. Also, not much can be done to improve the bearings without massive rebuilding and added cost. Users will simply have to be careful not to disturb the telescope while imaging. This includes wind protection, stable mounting of the equipment and avoiding touching the equipment while using it.
It is also clear that more complex and costly designs can and have been effected if significantly greater resources (money) are applied. There are some damping and control devices that can be brought to bear but these are complex to design and use.
One device that can improve damping of vibration is a mechanical damper. I have not seen a design that uses this technique on a telescope. They are very common on larger motor and engine mounts. Another is active adaptive damping on which I am working personally. (but at a slow pace) Still another is a method which does not damp the mechanical system, but adjusts the optical system to stabilize the image directly. This is a promising technique which we will soon see in practice. It is called Active Optics.
Stabilizing a telescope to one arc second, if that is required for good imaging, is very difficult. But it is a reasonable goal when imagers and seeing are of that same order.
I hope you find this information useful.
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