Introduction

If the PS and the SeS have different spectra, the will be offset in position in the night sky due to atmospheric refraction. The optical path length correction applied by the air delay lines will be different for each of the two stars, hence introducing an error into the differential astrometry measurements (note that this error is proportional to the offset of the main delay line from the point where the air paths are equalised). In order to subtract this effect, the spectral energy distribution of the correlated flux from each star will have to be accurately determined.