  A new method for estimating the angular power spectrum C_l from cosmic
microwave background (CMB) maps is presented, which has the following desirable
properties:
  (1) It is unbeatable in the sense that no other method can measure C_l with
smaller error bars.
  (2) It is quadratic, which makes the statistical properties of the
measurements easy to compute and use for estimation of cosmological parameters.
  (3) It is computationally faster than rival high-precision methods such as
the nonlinear maximum-likelihood technique, with the crucial steps scaling as
n^2 rather than n^3, where n is the number of map pixels.
  (4) It is applicable to any survey geometry whatsoever, with arbitraty
regions masked out and arbitrary noise behaviour.
  (5) It is not a "black-box" method, but quite simple to understand
intuitively: it corresponds to a high-pass filtering and edge softening of the
original map followed by a straight expansion in truncated spherical-harmonics.
  It is argued that this method is computationally feasible even for futute
high-resolution CMB experiments with n=10^6-10^7. It is also shown that C_l
computed with this method is useful not merely for graphical presentation
purposes, but also as an intermediate (and arguably necessary) step in the data
analysis pipeline, reducing the data set to a more manageable size before the
final step of constraining Gaussian cosmological models and parameters - while
retaining all the cosmological information that was present in the original
map.
