  We suggest a solution to an important problem of observational
helioseismology of the separation of lines of solar acoustic (p) modes of low
angular degree in oscillation power spectra by constructing optimal masks for
Doppler images of the Sun. Accurate measurements of oscillation frequencies of
low-degree modes are essential for the determination of the structure and
rotation of the solar core. However, these measurements for a particular mode
are often affected by leakage of other p modes arising when the Doppler images
are projected on to spherical-harmonics masks. The leakage results in
overlaping peaks corresponding to different oscillation modes in the power
spectra. In this paper we present a method for calculating optimal masks for a
given (target) mode by minimizing the signals of other modes appearing in its
vicinity. We apply this method to time series of 2 years obtained from
Michelson Doppler Imager (MDI) instrument on board SOHO space mission and
demonstrate its ability to reduce efficiently the mode leakage.
