To achieve the goal of a simultaneous detection of an optical--ray event the optical telescope must be already observing that field from where the GRB will be received. To do this the minimal requisite is a wide field optical telescope scanning the sky area under observation by -ray satellite. As an example we now analyze the following configuration: i) telescope: mirror diameter D=1 m, focal ratio f = 1:1, size of field , scale factor mm-1; ii) detector: two CCD arrays, each CCD featuring mm active area, 40000 rebinned pixels (with pixel size m, corresponding to about in the focal plane), quantum efficiency in the range , read-out time 1 s (equal to the exposure time, to allow double buffer image acquisition by switching between the two arrays).
We now evaluate the detection limit and the catching probability when such a telescope is scanning the detector field of view. We assume 1 s exposures and a 0.1-0.5 s shifting time from one position to the next.
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