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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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