Nordic Optical Telescope

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Measurements of M1 dynamics using the load cells

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;

  1. telescope parked a zenith,with power off.
  2. telescope parked at zenith with power off and all cooling fans off.
  3. telescope parked at zenith and the active optics bellow servo ran at a very slow (0.5 second) up-date rate.
  4. 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.

Figure 1:   Mirror vibrations, power spectra

Graham Cox, 13 March 2001.

Tim Abbott, AiC
Wed Mar 21 12:56:05 GMT 2001

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