On the measurement of atmospheric extinction by Rufener's method

The paper examines the influence of zero-point drift on the determination of atmospheric extinction measured by Rufener's method (1964) of M and D extinction stars. Two main results are discussed: (1) the error introduced by the drift is independent of the filter or of the value of the extinction coefficient itself, and (2) the error is mostly influenced by the deviation from the ideal pattern of observation; the ideal pattern means the first pair of measurements is to be performed when the D star is on the meridian and the last pair of measurements when the M star is on the meridian. Observations of M and D stars should be arranged closely enough to the ideal pattern.