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THE EFFECT OF THE LOCAL ATMOSPHERIC
ENVIRONMENT ON ASTRONOMICAL OBSERVATIONS

by LORENZO ZAGO

PhD thesis presented to the
Ecole Polytechnique Fédérale de Lausanne
Swiss Federal Institute of Technology, Lausanne, Switzerland

This is a hypertext version of the report of a dissertation which presents in a comprehensive manner the issues related to the interaction of a modern telescope with its local atmospheric environment. In particular the multifold effects of wind and local "seeing", which are key design drivers in the project of astronomical observatories are analysed. This work is largely derived from the author's experience on the design and engineering of the ESO Very Large Telescope (VLT) observatory between 1984 and 1993.

This draft document is offered as a service to the astronomical projects community in the hope that it can be useful to the development of new observatory projects. No responsibility whatsoever is assumed about the pertinence and accuracy of all data and results presented. Comments are welcome.

This hypertext document was translated from a LaTeX source by means of the Latex2htlm program by Nikos Drakos.  The presentation is somewhat less than optimal and it does not include several figures found in the printed report.  Interested persons are welcome to ask directly the author about these missing elements.
Copies of the integral report are also available from the author at a nominal price of 100 Euros plus mailing.

Revised edition - Copyright © 2010 by Lorenzo Zago.

Saving and printing of individual pages is permitted for research purpose only.

Lorenzo Zago, email, last updated August 3rd, 2010 

• Abstract
• Acknowledgements
• List of main symbols
• Introduction
  • General presentation
  • Scope and summary
  • Original contributions
  • The ESO Very Large Telescope
• The image quality of a telescope
  • Seeing
  • Thermal turbulence, light propagation and image size
    • The microstructure of the temperature field in the atmosphere
    • The microstructure of the index of refraction and its relationship with "seeing"
  • Effects of wind loads
    • Wind turbulence
    • Guiding errors
    • Wind buffeting on "thin" primary mirrors
  • The overall image quality and its budget
• Telescope enclosures
  • The requirements of telescope enclosures
  • The recent evolution of design concepts for telescope enclosures
    • Classical domes
    • The MMT and its rotating building
    • Improvements in conventional domes
    • The ESO New Technology Telescope
    • Average seeing at different telescopes at La Silla
    • The ESO Very Large Telescope
    • A summary of outstanding problems
• Telescope aerodynamics
  • Wind loading on telescopes
    • Classical domes
      • Measuring equipment and procedure
      • Results
    • NTT rotating building
    • Cylindrical enclosure (VLT)
    • Retractable enclosure
    • Synthesis
  • Wind loading on the primary mirror
    • Outline of the problem
    • Wind tunnel tests with the telescope exposed
    • Tests with the 3.5-m mirror dummy
      • Measurements
      • Main results
      • Relationship between pressure fluctuations and wavefront aberrations
    • Wind tunnel measurements of the pressure/velocity field on the mirror
      • Measurements
      • Pressure-speed correlation
      • Correction of wind deformations by active optics systems
    • Synthesis
• Local "seeing"
  • The relationship of the temperature structure parameter to the mean velocity and temperature fields
  • Dome seeing
    • Literature review
    • Causes of dome seeing
    • The "steady local air" dome effect
    • Scaling variables and similarity of dome seeing
    • An order-of-magnitude estimate of dome seeing
    • Mirror seeing
    • Analysis of telescope image quality data data
      • Data bases
      • Analysis procedure
      • HRCam data
      • FOCam data
      • Analysis summary
    • Seeing caused by heat generation at the secondary mirror unit
    • Seeing and natural ventilation
    • Synthesis
  • Mirror seeing
    • Physical description of mirror seeing
    • Free convection
    • Mixed and forced convection
    • The cold mirror case
    • Experimental studies
      • A mirror seeing experiment with a 4-cm mirror
      • Seeing tests on a 62-cm mirror
      • Seeing tests on a 254-mm mirror
      • Other laboratory measurements
    • Analysis and modeling
      • Mirror in free convection
      • Ventilated mirror
      • Active correction of mirror seeing
    • Synthesis
  • Numerical models
    • Seeing computation by means of a fluid flow numerical model
    • CFD for evaluating mirror seeing
    • Application to a 1.25-m telescope project
    • Synthesis
  • Surface layer seeing
    • The optimum height of a telescope base
    • Reduced scale simulation of near-ground seeing
    • Similarity rules for near-ground flow
    • Pilot test for direct measurement of seeing
• Systems Engineering
  • Engineering criteria and guidelines
  • Wind turbulence on the telescope
  • Wind buffeting on the primary mirror
  • Dome seeing
  • Mirror seeing
  • Random conditions and operational aspects
  • A statistical model of telescope image quality
    • Parameterization of seeing and guiding effects
    • Wind versus seeing for the primary mirror
    • Guiding errors
    • Computation of the CIR figure
    • Computation procedure
    • Results and discussion
    • Synthesis
• Conclusions
• References
• The enclosure of the VLT 8-m telescopes
• Aerodynamic measurements on mirrors
  • Measurements with the 3.5-m mirror dummy
  • Measurements in the NTT
  • Measurements in the inflatable dome
  • Wind tunnel measurements
    • Velocity measurements above the mirror
    • Differential pressure measurements across the mirror

   
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