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The dynamics of the NOT primary mirror were determined in May 2000 by
taking the power spectra of the image motion of several stars. To
confirm the finding a different method was later used to measure
directly the actual mirror vibrations using its load cells. With the
help of Ingvar, who through the TCS logged the values of the load
cells at a sample rate of 100Hz, spectra of their readings were
produced. The resulting spectra (see figure 1) all show the same modes
of vibration at 12Hz and 14.5Hz with some narrow lines at about 22Hz
and 49Hz, Several different conditions for the telescope were used
when making the measurements to try and identify the possible
excitation of the mirror oscillations, they were;
- telescope parked a zenith,with power off.
- telescope parked at zenith with power off and all cooling fans
- telescope parked at zenith and the active optics bellow servo
ran at a very slow (0.5 second) up-date rate.
- Telescope tracking.
For all the above conditions it was found that virtually identical
spectra were produced suggesting that the modes of vibration are
intrinsic to the mirror itself.
Mirror vibrations, power spectra
Graham Cox, 13 March 2001.
Tim Abbott, AiC
Wed Mar 21 12:56:05 GMT 2001