TORTORA fast camera


E.Molinari, S. Bondar, S. Karpov, G. Beskin, A. Biryukov, E. Ivanov, C. Bartolini, G. Greco, A. Guarnieri, A. Piccioni, F. Terra, D. Nanni, G. Chincarini, F.M. Zerbi, S. Covino, V. Testa, G. Tosti, F. Vitali, L.A. Antonelli, P. Conconi, G. Malaspina, L. Nicastro, E. Palazzi

INAF, Università di Bologna, Università di Perugia - Italy; Research Institute for Precision Instrumentation, Special Astrophysical Observatory, Sternberg Astronomical Institute - Russia

INTRODUCTION

The optical transients of unknown localization may be very short. For example, the rise times of flashes of some UV Cet-like stars may be as short as 0.2 - 0.5 seconds, 30% of GRBs have the duration less than 2 seconds, and details of their light curves may be seen on time scales less than 1 ms. Also, of great interest are the observations of very fast meteors which may be of extra-Solar system origin.
To study the variability of large sky areas on such time scales, it has been proposed [10,11] to use large low- quality mosaic mirrors of air Cerenkov telescopes. However, in [12, 13] we demonstrated that it is possible to achieve the subsecond temporal resolution in reasonable wide field with small telescopes equipped with fast CCDs to perform fully automatic search and classification of fast optical transients. Moreover, the scheme of two-telescope complex [14, 15] able to study such transients in a very short time after detection has been proposed.
Here we describe the design and implementation of TORTORA (Telescopio Ottimizzato per la Ricerca dei Transienti Ottici RApidi) camera mounted on top of REM robotic telescope at La-Silla Observatory (ESO, Chile), and a complete two-telescope TORTOREM system developed by our group.



Figure 1. Images of TORTORA fast wide-field camera in REM dome. Left without cover showing the inner scheme.

DESIGN OF WIDE-FIELD CAMERAS

The TORTORA camera is described here. Its technical characteristics are presented in Table 1, and images Ð in Fig. 1. Each camera consists of main objective, the image intensifier used to downscale and amplify the image, transmission optics and the fast low-noise TV-CCD matrix. TORTORA camera is installed on top of REM telescope, which has an alt-azimuthal mounting, while FAVOR has its own equatorial mounting.
TORTORA
Main Objective
Diameter 120 mm mirror
Focal length 150mm
Focal ratio 1/1.2
Image intensifier
Photocathode S20
Diameter 90 mm
Gain 150
Scaling factor 5.5
Quantum efficiency 10%
          
 
Field of view 24ˆx32ˆ
Time resolution 0.13"
CCD Matrix
Model VS-CTT285-2001
Frame size 1388 x 1036
Pixel scale 81"/pix
Exposure 0.13 sec
Pixel size 6.5 μm
Read-out noise 6 e/pix
Star FWHM 2.7'
The TV-CCD matrix of each camera operates in 7.5 frames per second regime with 0.128 s time exposure and gaps between frames negligibly small. The data from CCD is broadcasted through the gigabit local Ethernet network to the storage RAID array with 0.5 Tb capacity. The data flow rate for the system is about 20 Mb/s, and so the storage may keep the raw data only for one day, until next observational set. Also, the raw data are transmitted to the real-time processing PC operating the custom pipeline software under Linux OS. The pipeline performs the detection and classification of transient events of various types, and tries to recognize already known objects, by comparing the time and position of each events with catalogues of satellites and with star catalogue to minimize the number of false events due to stellar scintillations under bad weather conditions. Also, there are some other software subsystems, the most important is the controller process handling the interaction of wide-field camera with robotic telescope and initializing the auto-focusing process in the beginning of each observational night, and the compression of pieces of raw data related to detected events after the night. The overall scheme of all the software subsystems are shown in Fig. 2.

Figure 2. Schematic view of interoperation of various TORTORA subsystems with REM telescope.

RESULTS

The ability to find transient in a wide field gave its result with the recording of the onset of the Naked Eye Burst GRB080319B, when the afterglow began in the field of view of Tortora. The event is recorded in this movie and published in Racusin J. L.; Karpov S. V.; Sokolowski M. et al., Broadband observations of the naked-eye Gamma-ray burst GRB080319B, Nature, Volume 455, Issue 7210, pp. 183-188, (2008)


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