  High-resolution astronomical imaging at sub-GHz radio frequencies has been
available for more than 15 years, with the VLA at 74 and 330 MHz, and the GMRT
at 150, 240, 330 and 610 MHz. Recent developments include wide-bandwidth
upgrades for VLA and GMRT, and commissioning of the aperture-array-based,
multi-beam telescope LOFAR. A common feature of these telescopes is the
necessity to deconvolve the very many detectable sources within their wide
fields-of-view and beyond. This is complicated by gain variations in the radio
signal path that depend on viewing direction. One such example is phase errors
due to the ionosphere.
  Here I discuss the inner workings of SPAM, a set of AIPS-based data reduction
scripts in Python that includes direction-dependent calibration and imaging.
Since its first version in 2008, SPAM has been applied to many GMRT data sets
at various frequencies. Many valuable lessons were learned, and translated into
various SPAM software modifications. Nowadays, semi-automated SPAM data
reduction recipes can be applied to almost any GMRT data set, yielding good
quality continuum images comparable with (or often better than) hand-reduced
results. SPAM is currently being migrated from AIPS to CASA with an extension
to handle wide bandwidths. This is aimed at providing users of the VLA low-band
system and the upcoming wide-bandwidth GMRT with the necessary data reduction
tools.
