Calibrating the Telescope for the Received Flux

Joachim Köppen Strasbourg 2009

If we want to use the telescope data for quantitative measurements of the sun, satellites and the moon, we need to calibrate it in terms of the radio flux that enters the telescope. The numbers we get from the receiver are only a relative measure (in dB over 1 microVolt), and they pertain to the input of the receiver only. Thus anything that happens in the cable, in the LNB, and in the parabolic dish, we don't know ...

The designers supplied us with their Flux calibration report

We can use the thermal radiation of the ground - or the Holiday Inn hotel - as a calibration source: its temperature will be close to 300 K, or if we want to be more accurate or consciencious: between 273 K (for 0 deg C in winter) and 333 K (for 30 deg C in summer)! By pointing the telescope to that building, we fill the entire antenna beam with that radiation of a known temperature.

We use the following procedure, before and after we make an observation of the sun or a satellite. Because the signal we are measuring is dominated by the noise of the LNB, but is also affected by the receiver, whose calibration slightly drifts as the receiver circuitry warms up after switching on and may be affected by the action of the internal cooling fan, the background as well as all measurements are influenced by these changes. It therefore is essential to do the calibrations as often as possible, and as close as possible to the actual measurement!

This is what we must do: I suggest that you start the measurement, and then go through the steps, so that all the calibration data as well as your observational data is kept in one file. This is convenient, and there will be no confusion about what calibration belongs to what observations ...

Here is an example of two complete drift scans across the Sun, with the calibration measurements at the start, in the middle, and at the end. The background measurements are simply the sky before and after the passage of the Sun. It is quite obvious that both the background and the calibration levels changed over that time interval. So it is a good idea to get these measurements!

Please note that this plot was done before the repair of the telescope in 2007. Now the solar peak signal is usually 4 dB above the calibrator!

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last update: May 2013 J.Köppen