Hugh Thompson

Vibration budget for observatory equipment

Abstract. Vibration from equipment mounted on the telescope and in summit support buildings has been a source of performance degradation at existing astronomical observatories, particularly for adaptive optics performance. Rather than relying only on best practices to minimize vibration, we present here a vibration budget that specifies allowable force levels from each source of vibration in the observatory (e.g., pumps, chillers, cryocoolers, etc.). This design tool helps ensure that the total optical performance degradation due to vibration is less than the corresponding error budget allocation and is also useful in design trade-offs, specifying isolation requirements for equipment, and tightening or widening individual equipment vibration specifications as necessary. The vibration budget relies on model-based analysis of the optical consequences that result from forces applied at different locations and frequencies, including both image jitter and primary mirror segment motion. We develop this tool here for the Thirty Meter Telescope but hope that this approach will be broadly useful to other observatories, not only in the design phase, but for verification and operations as well.

Measuring transmission and forces from observatory equipment vibration

We describe measurements of both the vibration forces imparted by various types of observatory equipment, and the transmission of these forces through the soil, foundations and telescope pier. These are key uncertainties both in understanding how to mitigate vibration at existing observatories and for developing a vibration budget in the design of future observatories such as the Thirty Meter Telescope. Typical vibration surveys have measured only the resulting motion (acceleration); however, this depends on both the source and the system being excited (for example, isolating equipment results in less force being transmitted, but greater motion of the equipment itself). Instead, here we (a) apply a known force input to the pier from a shaker and measure the response at different locations, and (b) use isolator properties combined with measured acceleration to infer the forces applied by various equipment directly. The soil foundation and pier transmission can then be combined with a finite element model based vibration transmission analysis to estimate the optical consequences. Estimates of plausible source levels supports the development of a vibration budget for TMT that allocates allowable forces to the sources of vibration; this is described in a companion paper.

Equipment vibration budget for the TMT

Vibration from equipment mounted on the telescope and in summit support buildings has been a source of performance degradation at existing observatories, for adaptive optics performance in particular. To ensure that that the total optical performance degradation due to vibration is less than the corresponding optical error budget allocation, a vibration budget has been created that specifies allowable force levels from each source of vibration in the observatory (e.g., pumps, chillers, cryocoolers, etc.). In addition to its primary purpose, the vibration budget allows us to make design trade-offs, specify isolation requirements for equipment, and tighten or widen individual equipment vibration specifications as necessary. Defining this budget relies on two types of information: (i) vibration transmission analysis that determines the optical consequences that result from forces applied at different locations in the Observatory and at different frequencies; and (ii) initial estimates for plausible source amplitudes in order to allocate force budgets to different sources in the most realistic and cost-effective manner. The transmission of vibration from sources through to their optical consequences uses the finite element model of the telescope structure, including primary mirror seg- ment models and control loops. Both the image jitter and higher-order deformations due to M1 segment motion are included, along with the spatial- and temporal-correctability by the adaptive optics system. Measurements to support estimates of plausible soil transmissibility are described in a companion paper. As the detailed design progresses and more information is available regarding what is achievable at realistic cost, the vibration budget will be refined.

Experiments at the W.M. Keck Observatory to support the Thirty Meter Telescope design work

In order to validate various assumptions about the operating environment of the Thirty Meter Telescope (TMT), to validate the modeling packages being used to guide the design work for the TMT and to directly investigate the expected operation of several subsystems we have embarked on an extensive campaign of environmental measurements at the Keck telescopes. We have measured and characterized the vibration environment around the observatory floor and at certain locations on the telescope over a range of operating conditions. Similarly the acoustic environment around the telescope and primary mirror has been characterized for frequencies above 2 Hz. The internal and external wind and temperature fields are being measured using combined sonic anemometer and PRT sensors. We are measuring the telescope position error and drive torque signals in order to investigate the wind induced telescope motions. A scintillometer mounted on the telescope is measuring the optical turbulence inside the telescope tube. This experimental work is supplemented by an extensive analysis of telescope and engineering sensor log files and measurements, primarily those of accelerometers located on the main telescope optics, primary mirror segment edge sensor error signals (residuals), telescope structure temperature measurements and the telescope status information.

On the precision of aero-thermal simulations for TMT: revisited

Environmental effects on the Image Quality (IQ) of the Thirty Meter Telescope (TMT) are estimated by aero-thermal numerical simulations. The current study constitutes an update of the ongoing effort to minimize simulation time and to make the computation tractable with available computational resources, to understand the subsequent physical and numerical limitations, and finally to develop the approach to mitigate the issues experienced. In particular, the paper describes a mesh and time-step independence study as well as the parameters that influence the slope of the Optical Path Difference (OPD) structure function and the TMT Normalized Point Source Sensitivity Image Quality metric in the context of thermal seeing.

Ground layer studies for the alternate TMT site

The Observatorio del Roque de Los Muchachos (ORM) on the Canary island of La Palma has been selected as the alternate site for the Thirty Meter Telescope (TMT). Several potential locations on the summit needed to be investigated in terms of Ground Layer (GL) strength. Moreover, the presence of existing observatories necessitated a study of the interaction between these observatories and TMT. Lack of localized site testing and the nature of the terrain led to the use of Computational Fluid Dynamics (CFD) simulations combined with a seeing model for GL optical turbulence estimates. Three candidate locations for TMT at ORM were investigated under certain wind directions using steady-state simulations. For the most likely candidate the influence of TMT on two nearby telescopes, Gran Telescopio Canarias (GTC) and Telescopio Nationale Galileo (TNG), and vice-versa was also explored and conclusions were drawn.

Minimizing motor cogging and vibration for the Thirty Meter Telescope

The Thirty Meter Telescope (TMT) needs to maintain precise positioning of the optical elements to deliver unprecedented image quality. Any vibration from observatory sources must therefore be minimized; model-based analysis leads to maximum allowable forces from any individual source typically of order a Newton or less in the most sensitive frequency band. Careful attention throughout the design process is needed to ensure that these challenging requirements are satisfied. We focus here in particular on cogging forces from the azimuth motor drive. Each motor yields periodic tangential and vertical forces of order 100s of Newtons, with higher harmonics of the waveform potentially exciting telescope structural resonances. There are 56 drive motors, and appropriate phasing between them can ideally cancel most of the net torque or net vertical force. However, the moment created between non-collocated forces still produces image motion even if the net force cancels, and further, small errors in forcer positioning result in imperfect cancellation. We provide a general methodology for estimating total cogging forces and then present an example of the expected TMT performance impact from motor cogging placed in context with the larger challenge of demanding vibration requirement.

On the precision of aero-thermal simulations for TMT

Environmental effects on the Image Quality (IQ) of the Thirty Meter Telescope (TMT) are estimated by aero-thermal numerical simulations. These simulations utilize Computational Fluid Dynamics (CFD) to estimate, among others, thermal (dome and mirror) seeing as well as wind jitter and blur. As the design matures, guidance obtained from these numerical experiments can influence significant cost-performance trade-offs and even component survivability. The stochastic nature of environmental conditions results in the generation of a large computational solution matrix in order to statistically predict Observatory Performance. Moreover, the relative contribution of selected key subcomponents to IQ increases the parameter space and thus computational cost, while dictating a reduced prediction error bar. The current study presents the strategy followed to minimize prediction time and computational resources, the subsequent physical and numerical limitations and finally the approach to mitigate the issues experienced. In particular, the paper describes a mesh-independence study, the effect of interpolation of CFD results on the TMT IQ metric, and an analysis of the sensitivity of IQ to certain important heat sources and geometric features.

Systems engineering of the Thirty Meter Telescope for the construction phase

This paper provides an overview of the system design, architecture, and construction phase system engineering processes of the Thirty Meter Telescope project. We summarize the key challenges and our solutions for managing TMT systems engineering during the construction phase. We provide an overview of system budgets, requirements and interfaces, and the management thereof. The requirements engineering processes, including verification and plans for collection of technical data and testing during the assembly and integration phases, are described. We present configuration, change control and technical review processes, covering all aspects of the system design including performance models, requirements, and CAD databases.

Heat balance and thermal management of the TMT Observatory

An extensive campaign of aero-thermal modeling of the Thirty Meter Telescope (TMT) has been carried out and presented in other papers. This paper presents a summary view of overall heat balance of the TMT observatory. A key component of this heat balance that can be managed is the internal sources of heat dissipation to the ambient air inside the enclosure. An engineering budget for both daytime and nighttime sources is presented. This budget is used to ensure that the overall effects on daytime cooling and nighttime seeing are tracked and fall within the modeled results that demonstrate that the observatory meets its performance requirements. In the daytime heat fluxes from air-conditioning, solar loading, infiltration, and deliberate venting through the enclosure top vent are included along with equipment heat sources. In the nighttime convective heat fluxes through the open aperture and vent doors, as well as radiation to the sky are tracked along with the nighttime residual heat dissipations after cooling from equipment in the observatory. The diurnal variation of thermal inertia of large masses, such as the telescope structure, is also included. Model results as well as the overall heat balance and thermal management strategy of the observatory are presented.