National Radio
Astronomy Observatory

C.1.1 Long wavelength calibration

Synchrotron sources can vary over a light crossing time, so ones used as FD calibrators must have most of their emission coming from an extended region. The additional requirement that they be nearly unresolved therefore forces them to be distant, meaning that candidates which also have high apparent fluxes are quite rare. The following standards mostly share the same set of objects, and monitor their FDs every few years to account for variations. No interpolation is done between epochs, though - you are encouraged to choose the standard which observed your FD calibrator closest to the time you observed it at. The measurements are interpolated in frequency, however, using second to fourth degree polynomials of the frequency’s logarithm.

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