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:
As a consequence a simple parameterisation 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 parameterisation 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 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 indicate that this model does reflect the physics of the phenomenon. The analysis of different experiments allows also to validate two simple parameterizations for mirror seeing with respect to the surface-air temperature difference, with and without external ventilation.
However, the new knowledge elements brought here, as well as the ones that may come from a deeper insight of the various aspects of the telescope/environment interaction, will not by themselves make the design process of astronomical observatory more straightforward. Rather, they quantify more accurately a set of requirements that remain contradictory (e.g. the wind-versus-seeing dilemma mentioned in the introduction and through chapter ). As these knowledge gaps are overcome, the engineering of telescope enclosures shall also tend to a better structured process of concurrent engineering, in which there is a large scope for the system analysis and the optimization of the overall performance of the telescope+enclosure combination.
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 for a turbulent wind flow of 2-3 m/s and a guiding accuracy better than 0.05 arcsec with the top ring in open air. Then the best enclosure will be represented by the retractable dome illustrated in fig. .
Yet, even within the limits of the present state of the art, there is another approach to an optimized observatory, which consists in including in the engineering phase also all the 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 a apriori 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 also 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. When all the environment dependent error sources are reliable parameterized, the overall image quality can be predicted for any option in the observation program, which can then be scheduled 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 50 from the average wind direction, there will be only two ranges in azimuth, for a total of 70 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 concurrently to the general design of an astronomical observatory.