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Tuned Vibration Damper

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#1 Dick Jacobson

Dick Jacobson

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Posted 09 December 2012 - 01:31 AM

The strange looking device pictured below is a vibration damper that cuts the vibration of my telescope about in half. When the tube is bumped, the damper absorbs much of the vibration and damps it out.

The need for this device arose several months ago when I added a seven-focuser rotating secondary cage to my 14” Newtonian (see Seven focusers, one telescope). This about doubled the weight of the tube. Since I did nothing to strengthen my lightweight equatorial mount, it was no surprise that the telescope developed a severe vibration problem. It took eight seconds for the vibrations to die out after a “thump” on the tube. With a pair of dampers, one on each side of the tube, the vibration time was reduced to 3-4 seconds.

The damper is very simple to construct. It’s just a weight at the end of a wooden dowel. The dowel is firmly attached to the balance point of the telescope. I used a pair of identical dampers on opposite sides of the tube to maintain balance, also adjusting counterweight behind the mirror so the tube is balanced about the declination axis. A split-block clamp at the left of the photo attaches the dowel to the aluminum frame that is at the balance point of the tube.

I call this a “tuned” damper because the vibration frequency has to be adjusted so it is about equal to the natural frequency of the telescope’s vibration. There are three ways to adjust the frequency: change the position or amount of the weight, or the strength of the shaft. With the shaft pointing upward, it was easy to make adjustments by adding or removing large fender washers to/from the weight. I did this until I found the amount of weight that made the damper vibrate most vigorously after a thump on the tube. Later I replaced most of the fender washers with a steel cylinder. The weight is held in position by a pair of shaft collars.

The device is based on the physics of coupled harmonic oscillators. When two objects that vibrate at nearly the same frequency are coupled together, vibrational energy is passed back and forth periodically. In this case, vibration from the telescope is passed to the damper, then back to the telescope and so on. This isn’t exactly what I wanted. The goal is to damp out the vibration before it can be fed back into the telescope. This goal is achieved by adjusting the size and material of the shaft. I experimented a little with a threaded steel rod, but it worked poorly since the damping action was very slow. A wooden dowel has a lot more damping power. My limited experiments indicate that the total weight of the damper(s) needs to be about 5 percent of the weight of the tube in order for it to work effectively. When I used a smaller damper, it vibrated a lot but did little to reduce tube vibration.

Of course it’s best if the telescope doesn’t vibrate in the first place, and I have plans to strengthen the mount. But if you have a vibration problem, this simple and cheap device might be a better alternative to building or buying a heavier mount.

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