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Self-Calibration: Fine Tuning Your Image

Errors Left Behind

Self-Calibration is used to remove errors which remain after performing the regular calibration process. Regular calibration involves making occasional observations of a simple radio source of known structure to determine how the telescope responds to it. This then allows the astronomer to apply the telescope's response to the targeted source and turn its radio emissions into an image. The calibration process, however, is not perfect as it leaves behind errors in the image which can still be eliminated. The errors remain for two reasons.

First, the calibration source and the object under observation lie in separate parts of the sky. Consequently, the array is pointed at different parts of the atmosphere during calibration, then observation. Atmospheric conditions can vary considerably across the sky, affecting the incoming signals distinctly. Such directional differences typically show up as noise in the final image.

Second, it is not possible to observe the calibrator continuously as this would leave no time to observe the object of interest. However, the atmospheric conditions may fluctuate slightly while the object of study is being observed. If such fluctuations happen rapidly, the regular calibration process may not be able to correct for them adequately. Rapid atmospheric changes can add further noise to the final image.

Smoothing Out the Noise

Self-calibration offers a means to remove noise caused by spatial or temporal fluctuations in the Earth's atmosphere. The procedure is premised on the fact that such fluctations affect the response of each individual receiving dish in the array uniquely. Every measurement from a baseline involving a specific antenna is related back to that dish's signal response. In effect, self-calibration uses some of the data gathered by each dish to more precisely determine its response; another part of this data is combined with signals collected by the other dishes in order to create the image.

Image of Cygnus A : Before Self-calibration

This image of Cygnus A was produced after the deconvolution process, before self-calibration was applied. The blue and black speculations in the background represent noise.
JPEG Image (27.5K); Credits and Copyrights

Image of Cygnus A : After Self Calibration

The image to the left was created using the same data after self-calibration. Here the noise is much reduced. As a result, the faint trail of the jet connecting the central galaxy to the outer lobes is revealed.
JPEG Image (12K); Credits and Copyrights

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NCSA. Last modified 11/14/95