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Drifting lazily on a summer afternoon

Submitter: Paul Boven
Description: This past weekend the volunteers of CAMRAS, who are restoring the venerable Dwingeloo radio telescope back to working order, organised a weekend of observations. Our goal was to measure the performance of the telescope after all the work that has been carried out in the past year.

Escaping the oppressing heat and humidity outside, we pointed the telescope in a fixed direction, and watched a number of bright astronomical objects drift by. These driftscans were carried out with a bandwidth of 48kHz at a frequency 1296 MHz, which corresponds to a wavelength of 23 cm. The rotation of the Earth causes the objects to "move" by a maximum of 0.25 degrees per minute, depending on the source declination. (The Pole Star, at a declination of almost 90 degrees, stays approximately at the same position, and only rotates).

The top graph shows some multi-minute driftscans, with an integration time of 4 seconds. Using the known declinations of these sources we converted the time-data to angular offsets from the main beam, and by normalizing the power measurements we were able to determine the shape of the radiation pattern of the dish: a HPBW of 0.64 degrees (see bottom graph).
Copyright: Paul Boven/CAMRAS
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