5 A new non-conventional approach

Now we discuss the possibility of increasing the probability of detection and of gaining sensitivity. Concerning the first problem we should use telescopes with larger fields of view and, to raise the detection sensitivity, larger collecting areas. In our opinion the best way to solve the above problems is to use large mirrors but - this is a major point - the high resolution typical of optical telescopes is unnecessary. There are several kinds of such instruments: Cherenkov telescopes currently operating or under construction (WOIT: Weeks 1988; CAT: Punch 1995; CANGAROO: Lamb 1997; SUBURU: Tanimori et al. 1995; MAGIC: Lorenz 1995), radioparaboloids and military radars (covered by mirror film). Cherenkov telescopes are large mirror or multimirror devices used to take images of Cherenkov light showers from gamma-ray primaries in VHE and UHE energy bands (GraniteIII, Lamb et al. 1995); they have wide field and their resolution, not suitable for conventional astronomical image acquisition, could attain with little effort 1$^\prime$, enough for our purpose.

Assuming for the Cherenkov telescope D = 10 m, focal ratio 1:1, size of focal plane $1~{\rm m}\times1~{\rm m}$, pixels size = $5~{\rm mm}\times5~{\rm mm}=1\hbox{$^\prime$}\times1\hbox{$^\prime$}$, we get a detection limit at m_R = 18.5.

Widening the field of view from 3$^{\circ}$ to 6$^{\circ}$ square degrees (Lamb 1997) and at a slew rate of 6$^{\circ}$ per second the probability of detection of an OT of typical duration would be 40% if the telescope stands during each integration and of 87% if the telescope continuously scans the field (but with obvious software complications).

The use of arrays of fast Panoramic Photoelectric Detectors, as MAMA (Timoty 1988) and EBCCD (CERN 1997) or of multianode PMT (Hamamatsu catalogue 1999) to build the imaging device to be put on the focal plain canister could allow the observation of the OT fine time structure (flashes of about 10^-4 s have been detected in the light curves of GRBs).

Acknowledgements

This investigation was supported by the Russian Fund of Fundamental Researches (grant 98-02-17570), by the Educational-Scientific centre "Cosmion", by the Italian Ministry of Foreign Affairs, and by the University of Bologna (Funds for selected research topics).