Using high order reflections, double-crystal rocking curves less than 0.1 s of arc wide are achieved. To use them for double-crystal topography (Bonse and Kappler 1958, Bonse 1958, 1961) it is necessary to stabilise the operating; point on the flank of the rocking curve against variation Delta theta caused by temperature drifts. An automatic control was developed which maintains the relative orientation of reference crystal and sample about the theta axis constant for exposure times of up to several days with an accuracy of better than 10-2 s of arc. The controller comprises two stages. Coarse adjustment is accomplished by a stepping motor and fine control by a piezoelectric transducer.