Photodetachment of negative ions in combined laser and low-frequency fields is investigated. The time-dependent Green's function method is used for calculation of electron flux at a macroscopic distance away from the photodetachment source, typical for a photodetachment microscopy experiment. In calculating the electron flux, we use the stationary phase method for the time integral, equivalent to the semiclassical approximation, to compute the time dependent wavefunction. The stationary points t (i) 1 , i = 1, ..., n correspond to time instances of launching of classical trajectories arriving at the detector at a given space-time point (r, t). The number of trajectories n contributing to the electron flux at any point in the classically allowed space-time domain can be controlled by varying the switching interval of the high frequency laser which initiates the photodetachment process. The divergences inherent in the electron flux in the semiclassical treatment are removed by using the uniform Airy approximation near the caustics.