The ELT early vibration assessment and for other Giant Telescopes

Abstract

High vibrations levels are detrimental to telescope pointing performance. Unfortunately, the true (final) vibration level on the telescope structure can be measured only when the machine is commissioned, i.e. much after the design phase is over. In this context, to reduce engineering risk, it can be useful to assess telescope vibrations very early - also in the preliminary design phase - in order, if needed, to adopt suitable countermeasures (e.g.: vibration damping, structure stiffening, or vibration isolation). This is particularly important in giant telescopes, since their natural frequencies are typically low enough to fall next to many vibration sources (wind, pumps, bogies, electric fans, vortex shedding). EIE developed an integrated procedure to assess the vibration level at the telescope hosted units - mirrors and instruments. The procedure: (i) identifies the relevant vibration sources, (ii) evaluates the vibration level for each source at the generation point, (iii) transfers the vibration from the generation point to the hosted units, and (iv) combines statistically the vibration sources to get the final vibration level. This paper presents EIE integrated procedure for the vibration assessment, and it discusses the most relevant vibration sources to be taken into account in giant telescopes.