Abstract: We have studied the eventual resolution losses in a monochromator without an entrance slit induced by the thermal distortions on the first two optical elements of the beamline, namely two flat mirrors. The monochromator, a modified SX-700, will be installed in the BW3 beamline at HASYLAB. The first mirror is exploited as a low pass filter. the second is the deflecting mirror in front of the plane grating. The plane of incidence of the first (second) mirror is perpendicular (parallel) to that of the grating. The investigation was performed by combining ray tracing and finite-element analysis. The source employed in the calculations is the wiggler spectrum emitted by one of the beamline undulators. The first mirror can absorb power densities larger than 1 W/mm2 without affecting the resolution of the monochromator. When the peak absorbed power density in the second mirror is 0.1 W/mm2 the spectral resolution deteriorates by almost 40% Ver menos