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The optimum height of a telescope base

The problem of deciding the height of the telescope pier is frequently one of the most controversial in the design of the observatory facility. On the one hand one would like to set the telescope out of the possibly negative effects of the ground proximity. On the other hand the cost of the building increases rapidly for every additional meter of height: first the cost of concrete, steel and handling equipment needed for a higher building, and later the additional time and effort required for every handling operation, from the initial assembly of telescope and dome to the regular maintenance handling of mirrors and instruments.

When ample funds were available, the opinion of designers tended to be quite prudent and conservative with regard to the possibility of seeing from the near-ground layer and the telescope support was set as high as practical (which in many cases was about 30 meters). When budgets were limited, telescopes were set at more modest heights and eventually the feared consequences on seeing quality did not materialize or anyway were not quantified. The case of the ESO 2.2-m telescope mentioned earlier in section gif is quite typical in this respect.

In reality the optimal height for a telescope from the standpoint of seeing quality will depend on the overall optical performance aimed for the telescope as well as of the particular phenomena in the near ground atmospheric layer, which may vary at different sites and also within particular locations of a same mountain site. For a modern technology large telescope which aims at a overall FWHM quality of the order of 0.5 arcsec, it will generally be desirable that the mean seeing contribution of the near-ground layer does not exceed about 0.1 0.15 arcsec. A less ambitious telescope will accept easily 0.2 0.3 arcsec from the near-ground layer without significant loss of performancegif.

The local conditions of the surface layer on astronomical sites during night-time will of course vary but generally share two important characteristics: the turbulence intensity is low and the ground surface experiences a strong radiative cooling. Therefore the temperature gradient in the local surface layer is generally stable and we have seen (cf. page gif) that in those conditions the seeing will increase when the turbulence is augmented.

The seeing in the surface layer can be evaluated by means of measurements of the coefficient at a few points along the vertical of the site. From the interpolated profile the equivalent seeing FWHM can then be computed by equation (gif). Fig. gif shows some averaged profile reported from different sites.

All these profiles were measured on "virgin" locations, during the site selection studies of new telescope projects. It may therefore be interesting to analyze the profiles on an already built observatory.

Figure: Some reported profiles from different observatories.

[4]The data from La Palma are from the site survey study for the LEST solar telescope [Ortolani 91].

Systematic recordings of the temperature structure coefficient

at the La Silla observatory were taken by the author during 11 consecutive nights from Nov 20 1986. The measurements were taken by special micro-thermal sensors developed at ESO and described in [Sarazin 92], located at three heights (10, 20 and 30 meters) on the La Silla main meteo tower (see fig. gif, on the left hand). The tower is located on the leeward edge of a roughly flattened ridge about 50 m wide, well exposed to the prevailing north winds of La Silla and not directly in the wake of the other domes built nearby on the ridge. There is also an asphalted road just upstream of the tower and therefore the test location may be taken as a reasonable example of a built site.

The wind was about 5 6 m/s during the measurement nights, which also corresponds to the yearly night-time average at La Silla. The data were averaged over periods of one hour and cover 70 hours of astronomically useful night-time. Short periods of cloudiness which occurred during some of the nights were taken out of the data set. The values recorded at the height of 10 m are generally higher than those at 20 meters, which are very similar to the values for 30 meters as can be seen by the statistical summary shown in table gif.

Table: Statistical summary of night-time data.

A further analysis suggested that the difference 10-20 meters was linked to the wind velocity. This is well shown in fig. gif, showing a plot of the ratios and versus the mean wind velocity. One can may recognize two ranges of wind velocity: a range of low winds up to about 10 m/s where the means of and are respectively about 10 and Km

and the range beyond 12 m/s where is about Km at all heights. Thus strong winds mix the surface layer such that does not depend on the height from the ground.



Fig. gif shows the values evaluated for each set of measurements of the integral with as respectively 10, 15 and 20 meters, assuming a linear variation of . The equivalent seeing values are given in the table below.

Table: Statistical summary of integrated FWHM (arcsec) from height

It appears that the values measured in this case are significantly greater than those of fig. gif, although they are by no means alarming in absolute terms: a typical requirement that the mean seeing contribution of the near-ground layer does not exceed about 0.15 arcsec, would place the height of the opening (slit) of the telescope enclosure at about 11 m.

next up previous contents
Next: Reduced scale simulation Up: Surface layer seeing Previous: Surface layer seeing

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995