Interpretation of the Recorded Data
Joachim Köppen Strasbourg 2010
The Waterfall plot A good way to look at the data and judge its quality is the 'waterfall' map, which shows the fluxes in a false-colour map as a function of both frequency and time. This map is done in real-time by the telescope software, but it can also be constructed afterwards from the recorded data. In Excel we have to use a few small tricks:
The map below shows the waterfall diagram of an in-band interference, consisting of two signals. To bring out the details of the signals, we had subtracted from each spectrum the background determined for that instant, and also show only some part of the entire spectrum:
Subtraction of the Baseline Since the radio signal from the celestial sources are rather weak, the spectrum shows always some background signal, mainly from the noise generated by the receiver itself. There may also be continuum radio emission from the sky, and unfortunately also noise from the ever-growing electronic pollution by various electronic equipment, such as computers and their switched power supplies. This essentially frequency-independent noise power simply adds up to the received signals at all frequencies. Therefore we may deal with it by simply subtracting this background, or the baseline as it is called in radio spectroscopy.
In this way, one obtains the true spectrum of the source. Integration over all frequencies then gives the emission from the source.
Reduction to radial velocity For galactic and extra-galactic sources, the frequency is not the physically relevant of interessant quantity. It rather is the radial velocity. This is obtained from comparing the frequency to that of the natural frequency of the hydrogen line, namely f0 = 1420.406 MHz:
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last update: Apr. 2010 J.Köppen