A comparison of CMB Angular Power Spectrum Estimators at Large Scales:
  the TT case
  In the context of cosmic microwave background (CMB) data analysis, we compare
the efficiency at large scale of two angular power spectrum algorithms,
implementing, respectively, the quadratic maximum likelihood (QML) estimator
and the pseudo spectrum (pseudo-Cl) estimator. By exploiting 1000 realistic
Monte Carlo (MC) simulations, we find that the QML approach is markedly
superior in the range l=[2-100]. At the largest angular scales, e.g. l < 10,
the variance of the QML is almost 1/3 (1/2) that of the pseudo-Cl, when we
consider the WMAP kq85 (kq85 enlarged by 8 degrees) mask, making the pseudo
spectrum estimator a very poor option. Even at multipoles l=[20-60], where
pseudo-Cl methods are traditionally used to feed the CMB likelihood algorithms,
we find an efficiency loss of about 20%, when we considered the WMAP kq85 mask,
and of about 15% for the kq85 mask enlarged by 8 degrees. This should be taken
into account when claiming accurate results based on pseudo-Cl methods. Some
examples concerning typical large scale estimators are provided.
