LOFAR calibration challenges

The LOw Frequency ARray (LOFAR will observe at 20-200 MHz. At those frequencies, large ionospheric phase variations distort the observed brightness distribution. Fortunately, the imahe may be stabilized for long integrations by using bright radio sources in the sky. The downside is that LOFAR fields will be very crowded, which presents calibration challenges of its own. This is especially true for the bright and extended sources that enter via the relatively high sidelobes of the LOFAR station beams. An extra complication is that these beamshapes vary rather strongly in frequency and time. Altogether, LOFAR will require much more processing than existing radio telescopes, and has only just become possible with the new generation of computers. Even so, new processing techniques like 'peeling' had to be developed to speed things up by several orders of magnitude.