Waterstradt, T. Diete, W. Engblom, C. Schacht, A. Scheel, M. Weitkamp, T. JACoW Publishing Geneva, Switzerland We have developed a compact and rigid mechanical design for a liquid-nitrogen-cooled vertically-deflecting double-crystal monochromator (DCM) for the ANATOMIX beamline at Synchrotron SOLEIL. An in-situ differential interferometer setup directly measures the pitch and roll parallelism between the 1st and the 2nd crystal under operating conditions with liquid-nitrogen flow and at pressures below 10⁻⁸ mbar. Factory measurements at moderate LN2 flow rates show a stability of the relative pitch of 25 nrad RMS (0.1 to 10 kHz) and a first relevant resonant frequency of 175 Hz. At lower flow rates, still sufficient to dissipate several hundred watts of heat load, an angular stability of 15 nrad RMS is achieved.