  We present the new technique for the determination of the low-mass slope of
the stellar mass function from the pixel-space fitting of integrated light
spectra. This technique is the extension of the NBursts full spectral fitting
technique. It can be used to constrain the stellar initial mass function (IMF)
of compact stellar systems having high relaxation timescales (Hubble time or
more). We used Monte-Carlo simulations with mock spectra to test the technique
and conclude that: (1) age, metallicity, and IMF can be very precisely
determined in the first "unrestricted" variant of the code for high
signal-to-noise datasets (S/N=100, R=7000 give the uncertainty of alpha of
about 0.1); (2) adding the M/L information significantly improves the precision
and reduces the degeneracies, however systematic errors in M/L will translate
into offsets in the IMF slope.
