Conclusions

We have described in this dissertation the main effects of the local atmospheric environment on the performance of an astronomical telescope and presented experimental databases, theoretical analysis and parameterizations which should contribute to establish the engineering of telescope enclosures on a more solid knowledge base.

Three main topics were studied:

1. The characterization of the wind turbulence responsible for high frequency guiding errors
2. The turbulent pressure fluctuations on the primary mirror
3. The local seeing effects caused by the mirror, inside the enclosure and in the atmospheric surface layer

• The sharp edges of the enclosure slit are the main cause of the turbulence on the upper part of the telescope and of the induced guiding errors. The length scale of this turbulence is related to the slit width with a factor varying generally between 0.2 and 1, which brings the largest part of the turbulent energy in the frequency range between 1 and 10 Hz. In conventional oversized domes the large distance of the top ring from the slit edge and the absence of through-flow prevent effects on the telescope. On the more recent enclosures designed for through-flow to eliminate dome seeing and tightly fitted on the telescope, the only practical possibility of corrective action is given by windscreens with a permeability of 20%.

• The surface figure of the relatively thin primary mirrors of the new 8-m generation are quite sensitive to the turbulent pressure fluctuations caused by wind buffeting. The turbulent pressure field on the mirror surface has been measured and analyzed with respect to its effects on dynamic deformations and consequent optical aberrations. It was found that, over a wide range of enclosure arrangements, the spatial correlation of the pressure field is mainly determined by the mirror disk shape and that the resulting optical aberration is predominantly astigmatic.

  As a consequence a simple parameterization can be found between the average rms of pressure variations on the mirror surface and the total optical aberration. Simple relationships can also be found between local speed values and pressure variations on the mirror, but they will depend on the enclosure arrangement. In the particular case of the VLT cylindrical enclosure, a parameterization independent of the azimuth angle is found between the average flow speed on the mirror surface and the rms of pressure variations. These expressions are particularly useful for a parametric analysis of the overall wind+seeing effects of the primary mirror.

• The various seeing effects generated in the local atmospheric environment surrounding the telescope and its enclosure have been described and quantified with the help of experimental measurements and similarity analysis. We have analyzed in particular the seeing caused by free convection flow inside the enclosure, the mirror seeing, the effect of heat dissipation at the secondary mirror, and the influence of the atmospheric surface layer outside the telescope enclosure.

• Mirror seeing is the predominant component of what was previously called dome seeing. The mechanism of the seeing production on the mirror was elucidated. The seeing effect is generated in a thin region just above the viscous-conductive layer where the temperature fluctuations are largest and most intermittent. If its cause could be visualized, seeing would appear to come from a thin but very turbulent layer "floating" a few millimeters above the surface.

  The hypothesis that mirror seeing depends essentially in the surface heat transfer leads to extend to this case the similarity expressions used for the atmospheric surface layer. The agreement of computations of seeing by means of this model with the experimental data indicates that this model does reflect the physics of the phenomenon. The analysis of several experiments allows also the validation of two simple parameterizations for mirror seeing with respect to the surface-air temperature difference, with and without external ventilation.

• The similarity criteria for a wind tunnel simulation of the seeing effects of the atmospheric surface layer have been determined. The positive outcome of some pilot tests in the LASEN wind tunnels opens the possibility to apply reduced scale tests to the site and layout studies for astronomical observatories.

1. A more extensive and detailed analysis of the optical aberrations caused by wind buffeting on a large mirror.
2. The analysis of the mirror seeing effect with respect to both spatial and temporal frequencies.

   Both subjects would be particularly important in the development of extended active and adaptive optics systems.

3. The application of numerical CFD models in the design of astronomical observatories, comprehending all aerodynamic, thermal and seeing aspects in one computational model.
4. The study of the various similarity scales applicable to local seeing effects and the use of reduced scale tests in the engineering of astronomical observatories.

In absolute terms, the optimal enclosure without local seeing can only be made for an outstanding telescope, possibly still better than allowed by the present state of the art, at least when large 8-m telescopes are concerned. Essentially this better telescope should have a primary mirror designed to sustain a turbulent wind flow of at least 3-4 m/s and a guiding accuracy better than 0.05 arcsec with the top ring in open air. In such conditions the best enclosure will be represented ultimately by the retractable dome illustrated in fig. .

Yet, even within the limits of the present state of the art, it is possible to improve the quality of the engineering of astronomical observatories by including in the development phase also all operational aspects of astronomical observations. We have mentioned in chapter the random nature of the influence of local atmospheric turbulence and how these effects interact with the variability of the natural seeing and with an also random distribution of telescope orientations. We have used these considerations to propose a statistical approach for the performance assessment. In fact one may envisage a few steps further in that direction.

Not all types of observations require the best image quality but some can only be made if this is outstanding. This consideration has already prompted the idea of a flexible scheduling of observations as a function of the natural seeing conditions. We can envisage to extend the scope of flexible scheduling to take into account in a more comprehensive manner all the local parameters contributing to the image quality, such as the actual wind loading on the telescope observing any given sky object and mirror seeing predicted by processing the information on the respective temperature evolutions for mirror and ambient air. This approach emphasizes the importance of having reliable parameterizations for all the environment dependent error sources, which has been a main objective of this work. Indeed this will allow the observatory to predict and evaluate the overall image quality for any option in the observation program and therefore to schedule it appropriately for optimized observations.

For example, recalling from chapter that the mechanical turbulence on the telescope will be stronger when the slit azimuth is between 15 and 40 from the average wind direction, there will be only two ranges in azimuth, for a total of 50 on 360 which will generally be critical for guiding errors. Therefore it is certainly possible to avoid the critical range simply by choosing an appropriate observation timing. Similar reasoning can be applied to all other effects and, with the help of the parameterizations provided by the present work, lead to develop the criteria for an optimized scheduling of the observations as a main part of the general design of an astronomical observatory.