Daily Image

Click here or on the picture for a full size image.

Standing on the Shoulders of Giants

Submitter: Jan Noordam
Description: At this moment, for excellent scientific reasons, the world is indulging in a veritable glut of new radio telescopes, to be culminated around 2020 in the giant Square Km Array (SKA). Not many people realize that, without the full-polarization Radio Interferometric Measurement Equation (RIME), the calibration of these new telescopes would have been a messy affair at best, adding greatly to the overall challenge. Fortunately, in 1995, after half a century of radio interferometry, the RIME was formulated in the nick of time by Johan Hamaker, Jaap Bregman and Bob Sault (HBS, see image), in a series of classic papers:

Part I: Mathematical formalism (J.P. Hamaker, R.J. Sault and J.D.Bregman), A&A Suppl. vol. 117, pp 137-147.
Part II: Instrumental calibration of an interferometer array (R.J. Sault, J.P. Hamaker and J.D. Bregman), ibidem, pp 149-159.
Part III: Interpreting the IAU/IEEE definitions of the Stokes parameters (J.P. Hamaker and J.D. Bregman), ibidem pp 161-165.
Part IV: The full-coherency analogue of scalar self-calibration: Self-alignment, dynamic range and polarimetric fidelity. (J.P. Hamaker), ibidem, vol 143 no. 3, pp. 515-534 (May I, 2000).
Part V: Making matrix self-calibration work: processing of a simulated observation. (J.P. Hamaker), A&A 456, 395-404 (Sep. II 2006).

The early work was done in the general context of the AIPS++ project (see e.g. AIPS++ Note 185), another difficult but necessary step on the road to SKA. More recently, the RIME has been elaborated in a series of papers by Oleg Smirnov, who feels that he stands on the shoulders of giants (thanks to which, he can look in new directions, as the picture illustrates):

Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism
II. Calibration and direction-dependent effects
III. Addressing direction-dependent effects in 21 cm WSRT observations of 3C 147
IV. A generalised tensor formalism

Due to natural inertia, it has taken some time for the data reduction software to adapt. Even now, 15 years later, only CASA (formerly AIPS++), MeqTrees and BBS (the LOFAR production system) are based on the Jones matrix formalism of the RIME. However, now that the first new telescopes are beginning to generate data, the rate of evolution must increase. The RIME plays an essential role in the drive towards 3rd Generation Calibration (3GC), which deals with instrumental effects that vary as a function of direction. The most important 3GC effects are the ionosphere, and station beamshapes that vary independently with time and frequency. Inevitably, the RIME will be the new Lingua Franca of radio astronomical data reduction.

Copyright: Stefan Wijnholds
  Follow us on Twitter
Please feel free to submit an image using the Submit page.