Study of Vibrations at SOAR

Abstract

The 4.1-m Southern Astrophysics Research (SOAR) telescope in Chile resembles larger optical telescopes by its thin actively controlled primary mirror, built-in tip-tilt correction, lightweight structure, and adaptive optics (AO) turbulence correction. The non-stationary vibration of the SOAR optical axis with a frequency of 50 Hz and an amplitude reaching 20-30 mas has been detected by the AO system, degrading the quality of optical speckle interferometric data. Accelerometers revealed that the 50 Hz tremor, driven by an external source, propagates within telescope structure and is strongly amplified by the response of the fast tip-tilt mirror servo system, producing characteristic elliptical beam path. The AO system also evidenced periodic components at 47 Hz in the defocus (matching the frequency of fans in electronic racks and computers) and at ~65 Hz in the astigmatism. The latter is associated with structural resonances of the SOAR primary mirror support, apparently excited by the wind. Periodic tracking errors with typical frequencies of 0.5-2.5 Hz are mostly caused by periodic errors of the encoders, depending on their alignment, and can be amplified by the mount servo at fast tracking rates. Vibration characterization at SOAR informs similar studies at other telescopes.