Gain in detection limit (in magnitude at 5σ significance) with respect to nominal transmission as a function of separation for HiRISE without a coronagraph, evaluated for different transmission factors: two (dotted) and three (solid) times nominal transmission. The simulation is performed for a β Pictoris-like host star with a 1200 K planet and 2 h of integration time. The S/N is computed with a matched filtering approach (see Sect. 3.1) comparing the simulated spectra with the noiseless input planet spectrum.