Vibration measurements in the Daniel K. Inouye Solar Telescope

Abstract

The Daniel K. Inouye Solar Telescope (DKIST) will be the largest solar telescope in the world, providing a significant increase in the resolution of solar data available to the scientific community. In large ground-based telescopes, vibration of telescope optics caused by the telescope subsystems is typically the limiting factor of image resolution. 1 The impact of vibration increases with the resolution of the telescope and is therefore a much greater problem in long focal-length telescopes, such as the DKIST. In addition, vibration is a consumer of the adaptive optics image-quality error budget limiting the correction available for atmospheric seeing. In some cases, the adaptive optics might even amplify the vibration at higher frequencies. For all of these reasons, a vibration error budget is a critical component in any large telescope project, and a plan for active vibration management and mitigation is critical to the success of a large telescope project. In the design of a large telescope, finite element analysis is employed and this is historically the only effort put into understanding vibration issues. However, after the telescope mount is constructed and the instruments and ancillary equipment are more clearly defined, there are many opportunities to perform path analyses by directly measuring the low-frequency single- input-single-output (SISO) frequency response function (FRF) between vibration source locations and image motion on the focal plane. These measurements are carried out using inertial-mass shakers along with seismic accelerometers providing an accurate measurement of the image degradation that will be caused by vibration sources in various locations. This allows the designers to determine an appropriate vibration mitigation plan (if needed) long before the vibration source is attached to the telescope. These measurements have proven sensitive enough that they can be performed for equipment not mounted on the telescope structure but located in the telescope building or even in nearby buildings. In a previous paper, techniques were described for measuring vibration which is particularly challenging at frequencies below 10 Hz where accurate measurement requires several noise reduction techniques, including high-performance windows, noise-averaging, tracking filters, and spectral estimation. In this follow-up paper, the development of the DKIST vibration budget detailing the advantages of the shaker measurement technique is described, along with examples of testing performed on the DKIST structures currently under construction at the Haleakala High-Altitude Observatories site in Maui, HI.