Foreword
The EAS-TOP telescopes were designed, manufactured, erected, tested and commissioned by Simpro Componenti in the years 1994-1996, upon commitment by Ansaldo Energia that achieved a supply contract by the Italian Institute of Nuclear Physics (INFN).
The telescopes are installed in Campo Imperatore area at an elevation of 2 000 m a.s.l., just above the Gran Sasso National Laboratories (LNGS). Here, they were added to other four similar telescop of different supply. These eight telescopes are aimed at observing the EAS in Čerenkov light for the EAS-TOP experiment carried out by the Gran Sasso National Laboratories.
The EAS-TOP experiment
The LNGS web page EASTOP provides details on the EAS-TOP experiment. Interesting info are also provided by the presentation (in Italian) “High altitude experiments: CLUE, EAS-TOP, TTC, ARGO” available on the web (>>). The scope of the experiment was "to study the air showers that high energy cosmic rays produce when passing through the atmosphere" (as read in the Law Draft N. 1926 submitted to the Senate of the Italian Republic on 9 January 1997).
In short, this research deals with the study of the cosmic rays by using the gamma astronomy. An interesting description of the cosmic rays study has been made available on the web (see pdf file “A caccia di raggi cosmici” that can be downloaded) by the astronomer Silvia Vernetto (>>). The EAS-TOP experiment was conceived by Prof. Gianni Navarra (see the pdf providing a remembrance of Prof. Navarra).
The acronym EAS stands for “Extensive Air Showers”. A detailed description of the EAS is provided in the handbook “Principles of Radiation Interaction in Matter and Detection” by Claude Leroy and Pier Giorgio Rancoita (page 522), as well as in the online presentation from Prof. Bruno Borgia's lessons (>>).
Each telescope houses four Čerenkov atmospheric light detectors. Čerenkov radiation is generated by the primary cosmic rays when passing through the Earth atmosphere (on this regard see Prof. Galeotti presentation available on the web >>). Some additional info on the Čerenkov radiation is is provided in "Čerenkov light" page of this web site.
Performances requested to the telescopes and domes
The commitment concerned the turnkey supply of the domes and frames of the four telescopes, which were devoted to house four Čerenkov light detectors. The supply included also the electrical and control systems, and the related automation. The light detectors typically consist of four parabolic mirrors of 900 mm diameter each that, when required, can be replaced by just one single mirror of larger diameter.
The telescopes are of altazimuth mount (see figure 1 for a pictorial view of the altazimuth coordinate system) and have the fonction of pointing and following a point source (see the page “Altazimuth coordinates” in this web site). The maximum required rotation capacity around both azimuth and zenith axes was of ±140 degrees, with tracking possibility around the zenith. The minimum positioning speed about each axis was of 1 degree/s, with a maximum tracking speed of 0,005 degrees/s and the capacity to switch automatically from positioning to tracking. The pointing tolerance is of ±0,05 degrees with respect to the local meridian, and ±0,05 degrees with respect to the local vertical axis. The pointing tolerance during tracking is of ±0,05 degrees, as well as the static positioning tolerance. The rotation tolerance about the azimuth and zenith axes was of 0,01 degrees, equal to the orthogonality tolerance of these two axes. The azimuth axis (AA) supports were required to allow to regulate the axis horizontality within 0,01 degrees. The system was required to keep operating under wind speed up to 40 km/h (either continuous or gust). The telescope frame was therefore required to be so stiff as to ensure the required precision even under the wind speed of 40 km/h.
Each telescope is housed in a building composed of a 5 m x 4 m ca. basement made of reinforced concrete with a wall approximately 60 cm high and 30 cm thick along its perimeter. The basement supports a dome formed by two half-shells each composed of four articulated segments rotating around a common horizontal axis with respect to the fifth lowest segment fixed to the wall. This articulated system is such to ensure the protection of the telescope when not operating and allow the maximum observation field when in service. In order to fulfil this last goal, it was required that the top of each cover would not exceed 50 cm above the concrete wall when completely opened. This solution allows to point the telescope toward the horizon.
The design ambient conditions for the external structures are: maximum wind speed of 100 km/h, snow load as per 3 m thickness, and minimum temperature of -30°C. In rest conditions and domes closed, hermetic sealing to snow and water is required even under the maximum wind speed.
The construction
Figures 2 through 15 show the most signficant phases of the construction.
Fugures 2, 3 and 4 show the arrival on site of the domes with all segments packed together (figure 2), the placement onto the basement wall of the packaged half-shells (figure 3), and the arrangement of a telescope frame (figura 4).
Figures 5 and 6 show some phases of the adjustment of the moving and electrical systems of the telescopes.
Figure 7 shows a detail of the telescope motor; while figure 8 shows the dome's emergency button.
Figures 9 through 15 show the domes opened and closed during the commisisoning phase and upon the erection completion.
Figure 16 shows a picture of the finite element model used for the structural analysis of the domes.
