and wafer stages with respect to one another and the imaging system it is also desired to maximize the slew rates at which stages 12 and 14 may operate and to control their motion and position relative to one another interferometers 25 and 26 may utilize heterodyne interferometry that utilizes a source with a low split frequency of about 3 0 mhz this ordinarily would limit the slew rates at which they could operate for example with a double pass system the doppler frequency shift would be 3 0 mhz for a slew rate of 0 5 m s and thus would limit the slew rates to about 0 5 m s the object of the present invention is to be able to increase the slew rates of the reticle and wafer stages irrespective of whether the respective slew rates are greater than or comparable to the corresponding split frequency one alternative is to operate at the higher rates by increasing the split frequency of the source however in certain end use applications it is preferable to operate at low split frequencies split frequencies that are lower than the doppler shift frequencies this may be accomplished by generating an electronic interference signal heterodyne or superheterodyne which will monitor the difference at which the stages are moving if it is desired that they move with a harmonic relationship the signal can be arranged so that the differences in doppler shift frequencies essentially add up to zero when the stages are moving with the harmonic relationship therefore that difference in frequency will be much less than the split.