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Calibrating the Array

Arrays must normally be calibrated to reduce errors caused by atmospheric and instrumental instabilities. Imagine trying to see a car a distance down the road on a hot sunny day. The hot air rising off the surface of the road wavers, causing the car's image to shimmy. Similarly, variations in atmospheric conditions cause stars to appear to twinkle when we look up at night.

Atmospheric instabilities make it difficult and sometimes impossible to obtain useful data. Adding to this difficulty, spurious signals in the array's complex instrumentation can drown out the incoming signals from space. To obtain images free of such "noise," astronomers need to correct for both types of extraneous signals. Doing so requires calibration.

During calibration, the array is pointed at a familiar, simple and intense radio source such as a quasar. Since the characteristics of the quasar's radio emissions are well known, discrepancies between what astronomers predict the array should measure and what it actually measures at a given time can be quantified. Knowing the magnitude of such errors, astronomers can subtract them from the data collected during an observing run. The goal of course is to image radio sources of unknown complexity as precisely as possible. Accurate calibration is essential for producing high-quality images.

Compare these images:

M82: Calibrated and Non-Calibrated

The image on the right was produced with uncalibrated data and contains no discernible features. Calibrated data was used to generate the left-hand image in which the overall shape of M82 is clearly visible.
JPEG Image (36K); Credit and Copyright

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NCSA. Last modified 11/14/95