Solar Observations with ESA-Haystack


Joachim Köppen Strasbourg 2010


Observations of the Sun are quite important, not only because of the Sun, but also to measure important parameters of the telescope itself. At the frequency of 1420 GHz the Sun emits a continuum radiation, so we do not see any spectral feature, but the received power is simply increased, when the telescope points to the Sun. This radiation is due to the hot gas which makes up the Sun. However, at radio frequencies the emission does not come from the photosphere that we observe in the visible wavelength range, but from the hotter layers above the photosphere: the Corona is composed of very thin gas at temperatures of a million K.

The other aspect is about the telescope: Since the empty sky provides radio wave emission only at negligible intensity, the signal that we measure is essentially the background noise from our receiver. Comparing the solar signal with that of the sky we obtain a measure of the sensitivity of our instrument: The larger the ratio of the solar and sky signals is, more sensitive is the system. We express this more quantitatively as the System Temperature.

The third aspect is also technical: We can determine the width of the antenna pattern, i.e. the Half-Power Beam Width (HPBW) which describes also the angular resolution possible with the antenna. This is done by letting the Sun - which we use as a source with a constant signal - pass through the antenna beam, and measure the time it takes. With the knowledge of the Earth rotation's angular velocity we convert the time interval into an angle, the HPBW.

Observational procedure

After starting up the system, we recommend to follow this sequence:


Analysis

Let's suppose that we use Microsoft Excel to do the interpretation of the data. Then we recommend to follow this sequence:


You might wish to do a more sophisticated analysis by fitting a simple model to the data:


Our results

Our first solar observation was done on 18 june 2009, the day of First Light:

This was done as decribed above. At this time, we operated the receiver without any bandpass filter, because we did not yet suffer from strong interference. Therfore, the results were much better: For that day, we also can use the Solar Indices Bulletin from NOAA. It shows that the solar flux was very constant throughout the day, and the flux measured at noon UTC by the observatory on San Vito Island (SVI) at 1415 MHz was 54 SFU (solar flux units ... 1 SFU = 10000 Jy). Our measured solar antenna temperature of 490 K would be consistent with an antenna efficiency of 60 percent (see System Temperature for more details).

Now that we have installed a narrow band-pass filter to protect the receiver from interference caused by transmitters operating on frequencies other than 1420 MHz, the unavoidable losses in the filter brought up our present system temperature to about 1000 K. This means that we need to do some more work to optimize the system!


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last update: July 2013 J.Köppen