  Dense aperture arrays provide key benefits in modern astrophysical research.
They are flexible, employing cheap receivers, while relying on the ever more
sophisticated compute back-end to deal with the complexities of signal
processing required for their optimal use. Their advantage is that they offer
very large fields of view and are readily scalable to any size, all other
things being equal. Since they represent "software telescopes", the science
cases these arrays can be applied to are quite broad.
  Here, we describe the calibration and performance of the AARTFAAC-12
instrument, which is composed of the twelve centrally located stations of the
LOFAR array. We go into the details of the data acquisition and pre-processing,
we describe the newly developed calibration pipeline as well as the noise
parameters of the resulting images. We also present the derived radio source
counts at 41.7 MHz and 61 MHz.
