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D. Mancini (1), A. Auricchio (1), S. Ortolani (2), I. Porceddu (3), V. Zitelli (4)
In order to obtain a better astronomical characterization of the Galileo Telescope site, a tower to hold a Differential Imager Motion Monitor(DIMM) was erected in the second half of 1993 some 100 m to the West of the foreseen dome location by Technology Working Group (TWG) of the Capodimonte Astronomical Observatory (Naples).
The tower has proven extremely reliable, and has been installed also for other Institutions as ESO for the VLT at Paranal, and IAC and ING at the Roque de los Muchachos Observatory (RdM).
A DIMM built by the Instituto de Astrofisica de Canarias was installed by TWG on the tower in the fall 1993, and there operated till August 1995.
A fully automated TNG DIMM, was built by TWG in collaboration with ATEC (Advanced Technology Company). The DIMM will be installed on the tower in the next months; its main improvements with respect to the other DIMMs are in the system management, mounting, pointing and tracking performances, and in the possibility to work in a fully automated way with self testing devices and diagnostic routines.
The TNG DIMM system basic idea is that no assistant is required to carry out its monitoring function, for general operation economy. As a consequence, high system reliability in terms of quality and continuity of the service has been reached. Thanks to TWG and ATEC experiences a fully automated system has been designed whose general scheme is shown in Fig.1.
The effect of wind, ice and lightning has been taken in due consideration in order to reduce the risk of faults. The system is divided in two main parts: the DIMM instrumentation installed on the tower, named SYS#1, and the user interface installed in a nearby Galileo office, named SYS#2.
SYS#1 is devoted to automatically manage all the basic activities. It is powered by means of an high efficency solar panel unit designed to supply the system for several days in case of severe weather conditions (up to 15 days). In this case the system will not work but the basic diagnostic functions will be under control. In case of longer periods the system provides an automatic shutdown to prevent damage to the batteries.
SYS#1 is provided with a local small meteo station mainly used to support the DIMM basic functions and also to provide a basic redundancy of the most important meteo information.
This unit is directly managed by the controller, while the main meteo station is located on a 15m high dedicated tower, far from the DIMM and connected to the DIMM control system by means of an optocoupled serial link.
The main meteo station is self powered by means of a local solar panel unit in order to reduce the risk of fault in case of lightning. The optical system is based on the use of a 8 inch Celestron Telescope.
Two prisms have been installed on the entrance pupil in order to obtain two different spots of the same star. A sensitive non-intensified CCD camera is mounted directly on the telescope focal plane.
The data flow between SYS#1 and SYS#2 is obtained by means of two full duplex radiomodem operating at 500MHz frequency, simulating a standard RS232 serial connection. SYS#2 updates two dedicated accounts in the local network (IACNET) with the last seeing data, periodically and/or when the network is available. At the same time SYS#2 is a local database allowing the user in the RdM to draw seeing data by means of a conversational software tool.
Each set of data is related to the absolute time of the acquisition, and reports at the same time seeing mean value and meteo data. The time rate presetted at 1 sec depends on the number of exposures used to extract the seeing mean value, but it can be modified. In any case the system provides the users also with a separate file of all the spot positions acquired every night, providing therefore the possibility of a variety of scientific computations using the most elementary information element. Like SYS#1, SYS#2 is Built in Test (bite) provided and is powered by means of a UPS (Uninterruptable Power System) checked by the main diagnostic task in order to prevent shutdowns during the phase of data exchange between SYS#1 and SYS#2.
A technical account allows to control the diagnostic files and provides a remote control of the complete system if necessary. It also allows to change remotely the software tools in the system, allowing the operators to upgrade the system performance.
Because the differential image motion uses a direct measurement method, in the sense that all parameters involved are perfectly measurable and no reference is needed, the main calibration lies in the estimate of the instrumental noise and system limitation.
The uncertainty in the determination of the relative positions of the centroids introduces an error appearing as a noise of the standard deviation which is not correlated with the true motion.
The accuracy of the centroid algorithm measured in laboratory on two fixed spots corresponds to an equivalent systematic error in both directions of the measurement of +/- 0.005arcsec. The exposure time is regulated by means of an electronic shutter from 0.01 msec to 20 msec.
This regulation is necessary to reject the main drawback of the differential method, namely the necessity to freeze the high frequency motion in case of turbulence in regions of the atmosphere with high wind speed. The regulation is automatically operated by the main computer in order to obtain at the same time a sufficient signal level and the minimum exposure time.
This regulation does not affect the instrumental error, because the centroid computation method doesn't critically depend on the spot illumination. The optical system scale has been measured by means of the telescope axis low error encoders. The scale measurement error has been eliminated with the average of several measurements obtained with different axis positions.
The automatic dome resulted the most critical subsystem. It must be able to open and close correctly in case of strong wind and ice, and at the same time it must be very light. A dome smaller but similar to the first one designed and installed by TWG on the IAC towers has been built for the TNG DIMM and successfully operated.
Tests on each subsystem have been performed by TWG. Fig.2 shows a run of seeing measurements of the sky above the Capodimonte Observatory.
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