Friday, 4 October 2013

Changes in frequency

As our regular readers will know, KAIRA is constantly monitoring the radio sky as part of its routine observations. We automatically update the image on the right-hand side of the web log showing a recent view (updates are typically made each hour).

Recently we made a change to the experiment which meant the image looks a bit more smeared out. This change was noticed by some of our regular viewers, who asked us why.

The reason is that the new observations use a different frequency. In fact, it is nearly half that of the old observation set. The resolution of a radio telescope is roughly governed by the size of the telescope and the wavelength. Very roughly, this is:

resolution (in radians) = wavelength  /  diameter

where 1 radian = approx 57 degrees, and the wavelength and diameter are measured in the same units.

The wavelength of the radio waves is related to the frequency:

wavelength = speed-of-light  /  frequency

If the frequency is in hertz, and the speed of light is in metres per second (specifically 3 x 108 m/s), then this gives the wavelength in metres.

The numbers are not too important. What is, though is that as the frequency gets higher, the wavelength gets shorter. And, as the wavelength gets shorter, the resolution gets finer.

We've made an observation to show this...




Watch for the frequency in the lower corner. Wait until it gets to about 18 MHz, at which point the main sources of "Cas A" and "Cyg A" snap into view. Then, watch as the frequency goes up, our determination of their positions improves. In other words, and the frequency goes up, the resolution gets better!

The very start and very end of the video are because there is insufficient signal to noise. However, between 18 and 80 MHz, it is mostly okay (apart from the odd burst of radio frequency interference).

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