August 2005

Vibration Analysis of a Quadruped Telescope Pier


L. A. Layton
Northern Illinois University, Department of Physics, DeKalb, Illinois, 60115
llayton1@san.rr.com

Abstract

A vibration travels through the quadruped telescope pier at Davis Hall Observatory. As a result, visual image shift is observed when viewing objects through the 14-inch Schmidt-Cassegrain telescope. Objects appear to oscillate up and down, more so when viewing near the horizon, and less so when viewing at angles closer to zenith. Isolating and eliminating the vibration are further complicated by the instrumentís location. The telescope pier is situated on the ninth floor of a nine-floor building and rests on top of a 4-foot-thick cement slab. The slab is supported by steel beams on the eighth floor, which couple it, the pier and the telescope to the rest of the building. Other factors to consider are that labs within the building are vented on the fifth floor, and that heating/air-conditioning units are located on the eighth floor, approximately 20 feet away from the cement slabís supporting beams. To date, attempts to isolate the pier from the source of the vibration have been unsuccessful. Methods of isolation attempted include placing different materials between the pier and the cement slab, placing two-inch-thick rubber pads underneath the pierís four legs, and fitting the pierís four legs with vibration-control mounts.

Introduction

Since the June 2000 replacement of Davis Hall Observatoryís 16-inch Nishimura, Newtonian-Schmidt telescope with a new 14-inch Celestron, Schmidt-Cassegrain telescope at Northern Illinois University, objects viewed through the eyepiece are observed to oscillate up and down continuously.

The vibration was observed the very first day the telescope was placed on top of the PierTech, custom-made pier on November 20, 2000. Because the PierTech pier was constructed in a manner that allowed for horizontal movement of about three centimeters side to side at its top, it was assumed that was the cause of the vibration. Piers should never move side to side at all, let alone several centimeters. Although the PierTech pier was flimsy, poorly constructed, and sits in Lab 101, Faraday building to this day, it was not the sole source of the vibration.

Subsequent to the PierTech pierís removal, Phil Stone in the Physics Department Machine Shop constructed and installed a quadruped steel pier. The frequency of the vibration was observed to become smaller, but it still remained and remains.

Because taking long-term CCD images and photographs of celestial objects at varying angles of observation is desirable, and because an oscillating object is difficult to image or photograph, attempts have been made to damp the unwanted vibration traveling through the pier.

Scope of Project

The focus of this project is to eliminate the vibration that is traveling up the telescope pier by either isolating the instrument from the vibration or damping the vibration before it reaches the telescope pier.

Observations

Video clips of several seconds each have been captured digitally, using one of the gargoyles on top of the newly remodeled Altgeld Hall as a target. These video clips serve as a baseline, so that any future video clips recorded after alterations to the telescope pier have been made will give a quantifiable determination of whether the vibration has been eliminated, damped, or amplified. Video clips are archived on the Davis Hall Observatory dome pc, saved on the Ďvideoí section of the hard-drive.

Using a ground-based target object allows for recording the vibration at angles near maximum amplitude, and makes it possible to take data during daylight hours.

The oscillation does not appear to die out gradually, and is observed to remain at the same frequency, regardless of whether celestial or ground-based objects are viewed. The oscillation, however, is noted to diminish somewhat when viewing at angles of 45 degrees and greater above the horizon.

The wavelength of the vibration is observed in the visual range, which is between 350 and 750 nanometers. The frequency of the vibration is visually estimated to be somewhere around 25 cycles per second (25 Hz). The angular frequency of the harmonic motion is two times pi times the frequency, and that equals about 157 radians per second in this case.

Damping Attempts

Several attempts have been made to damp the vibration. The first of many was to grout each of the pierís four feet to the existing cement slab in order to eliminate the possibility that the pier was oscillating back and forth on top of the cement slab. A very slight improvement in image quality was observed, but image oscillation was still considered to be severe enough to warrant further attempts.

The next try at improving the pierís rigidity involved adding metal bars between the two sets of legs of the quadruped pier. The thinking here was that the bars would increase the rigidity of the pier and thus damp the vibration.

It was requested that the metal bars be attached to all four sides of the pier, each at a 45-degree angle, and with two sides having the bars attached higher than the other two sides (asymmetrically), but the bars were installed horizontally at two differing heights. There was no noticeable difference in the vibration of objects seen through the telescopeís eyepiece.

In keeping with the rigidity theme, wooden trays were added on top of the two sets of metal bars in an attempt to couple the four legs of the pier together. Convenience was also a factor. It is convenient to have a place to put cameras, eyepieces and accessories. Still there was no appreciable difference in image quality.

Another attempt at damping the vibration involved filling each of the pierís four legs with silica sand. Again, next to no improvement was noticed in image stability. Overall, the image still oscillated at about 25 Hz when viewing celestial objects or ground-based gargoyles through the telescopeís eyepiece.

The last try at vibration damping was to add mass to the instrument. A 3/4-inch steel plate weighing in excess of 250 pounds was placed underneath the pier and bolted to its four feet. This actually made the vibration worse. Subsequently, it was determined that the mass should have been added to the top of the pier to achieve better results, but this option would not work physically.

Isolation Attempts

After attempts at damping the vibration proved unsuccessful, isolation of the vibration was next on the list of things to try. Because the instrumentís location is nine stories above the ground, at the top and center of the building it is built onto, there is no way to attach it directly to the ground, as nearly all telescope piers are (now we know why).

It is possible, even likely, that sources of vibration exist in the building and travel up through the infrastructure, passing through the steel beams that support the cement slab to which the telescope pier is attached. In this case, coupling the telescope pier securely to the slab should not make any noticeable improvements, which is what was observed.

Isolation of the pier from the building should eliminate the vibration. To this end, materials were placed between the steel plate on the bottom of the pier and the cement pier. Isolating materials included 3-inch-thick layers of cotton blankets, large bubble wrap normally used for packing, and rubber sheets. Each material was tried individually, and then in combination. No material placed between the iron plate and the cement slab had any effect on the vibration.

The iron plate was removed, and four vibration-damping rubber feet were placed between the cement and the pierís four metal feet. The rubber was semi-flexible and attached to the pierís feet with spray adhesive. No improvement was noted.

Results

The results of this project are that, to date, no attempt at damping or isolating the vibration that exists in the telescope pier has met with success. The vibration persists, and its quality remains relatively unchanged.

As would be expected, the vibration is more pronounced when viewing objects at higher magnifications, and less noticeable when viewing objects at lower magnifications. Using a 6.4 mm eyepiece yields a magnification of 611 times and results in an oscillation that is more noticeable than when using a 32 mm eyepiece, which produces a magnification of about 122 times and results in a less noticeable oscillation, when viewing the same object.

The oscillation is less noticeable when looking at a large object like the sun or moon through a 32 mm eyepiece (122 times magnification), and there is very noticeable jumping up and down of smaller objects like star clusters when viewed through a 12 mm eyepiece (326 times magnification). Visual viewing of planets is less than ideal. Mars, Jupiter, Saturn and Venus all appear to jump up and down periodically.

Although different magnifications result in the vibration being less or more noticeable, its frequency does not appear to change, or settle out with time. It is always there.

Conclusions

No CCD or photographic imaging of any targets other than the sun and moon is possible without substantial blurring taking place in the image. It is analogous to taking a photograph of a 50-yard dash using 100-speed film. Photographing faint objects at high magnification requires steady tracking and no vibration.

A large amount of mass could be added to the pier (in excess of one thousand pounds) in order to lower the frequency of vibration permitted to travel through the pier. Because the frequency of vibration is equal to the square root of the spring constant k divided by the mass of the pier, adding mass should make the vibration appear smaller, or eliminate it altogether. However, without knowing the exact frequency of the vibration, this also would be a shot in the dark. Adding 1,000 lbs. of mass is a nearly irreversible, time-consuming job. Cement and water would have to be carried by hand up four flights of stairs. This approach should only be taken upon the advice of a qualified mechanical engineer with vibration damping experience.

After finding only two papers written on the subject (1, 2), and neither of them dealing with a pier that is not directly connected to the ground, I conclude that there is no way to eliminate the vibration experienced at Davis Hall Observatory without a modal analysis being done. An accelerometer, an FFT analyzer, and analysis by a qualified mechanical engineer are needed in order to perform such a modal analysis.

References

Avitabile, Peter et al. (2001). Modal and Operating Characterization of an Optical Telescope. Sound and Vibration. June: 20-27.

Eaton, Joel A. (2001). Structural Analysis of the Telescope Mount. Tennessee Technical University [online]. November: 1-11.