For the first time I made a continuum observation from radiosource in our Galaxy. The supernova remnant Cas a.
But what is a continuum observation.
Continuum sources produce steady, broadband noise. Broadband noise, also in a radiospectrum. In an observation of it, we can measure the signal strength of this broadband noise.
But what is a supernova.
A supernova is an explosion of a large star(sun) in our Galaxy. This explosion happens in the starsystem Cassiopeia. This explosion starts with a very large image of light. The brightness of a lot of suns together. After this explosion a supernova remnant exist. Much gas is throwing away into the galaxy. A kind of ring of gas arises then. And that at a distance of 11000 light years away from us. In the picture below an röntgen image of the supernova remnant (photo NASA). The X-ray image shows an expanding shell of hot gas produced by the explosion. But supernovae are most common in the universe. The exist about every 10 seconds.
Cas A produces not only x-ray radiation, but also a large radio noise emission with a very broad spectrum, from 1 GHz to upwards in frequency. And is one of the strongest radio sources.
At CAMRAS, the manager of the radio telescope of Dwingeloo, they do many radio astronomical observations of our galaxy. Not only continuum observations, also observations of pulsars etc. Pulsar observations are often used in a demo to the public during summertime.
So sometimes also from the supernova Cassiopeia, Cas a. With the 25 meter telescope, they align the antenna within 0,5 degree to a that radiosource. And track it constantly. Because we have to deal with the rotation of our earth.
As a volunteer of CAMRAS, it was possible to make an observation on my own with that telescope. In the past I did that with observation of the H-line of our galaxy. So now I like to do this with a continuüm observation of Cas a. At a frequency of 1296,200 MHz.
What did I use for this observation:
A RTL SDR V3 dongle as a receiver. The HF input connected to the telescope antenna.
The RTL SDR connected by a USB cable to the USB input of a PC laptop.
Used software is SDR# for the SDR dongle and SpecLab for measuring the signal. The SpecLab software contains a plotter program. With the plotter we measure the the relative field strength against time. SpecLab receives a noise audiolevel from the SDR and puts the level in this plotter.
The audio from SDR# will be leaded to the SpecLab program by a program Virtual Audio Cable. so no need for external audio in- and output entries. Most laptops don’t have them both. only an external audio output
How do we do the observation of Cas a.
We do it with the aid of the 25 meter radio telescope of Dwingeloo as the antenna.
First we start up SDR #. We tune the receiver to 1296,200 Mhz. The gain of the SDR to half shell. We set up the audiolevel at a correct level.
After this, we start up SpecLab. In SpecLab we do the necessary settings. How the settings are, depends from the signal strength of the radiosource to the HF input of the SDR. Then we start up the plotter. In the plotter we do several settings, like settings in the tab “watchlist”. And in “horizontal” tab we make the correct setting for the relative field strength scale in the plotter screen. Also in the tab “horizontal” the scrolling speed of the plotter. It is all a bit of trying it out, when you do the observation for the first time.
Then we start up the radio telescope and align it to Cas a. When aligned exactly to the az/el coordinates, we see then a certain increased strength level on the plotter screen. If unsufficient, increase the gain of the SDR a bit. But in such a way, that the signal/noise ratio is not getting worse.
When sufficient, and all the settings are correct, we point the telescope about -3 degrees in azimut to the left of the radiosource. We see that the signal strength level is decreasing in the plotter screen.
Now we erase the memory of the plotter, the plotter start up again, a new offset trace is appearing.
After a smal delay, we align the telescope to a new position in azimut of 3 degrees right of the source. (+3).
At the plotter screen there will be appearing a maximum of signal strength. A small top. so first level low, then a maximum and after that the first low level.
This maximum is the exact position in azimut of Cas a. The correct elevation will be tracked constantly by the telescope. His position would be changed a bit because of the rotation of our earth. So off course the azimut.
In the plotter screen I saw a signa strength of about 2 db above thermal noise (offsetlevel) . That could be better. A 6 db level is possible. By increasing the sensitivity by increasing the gain of the SDR we could reach that level. I will try it out another time.
It is amazing that we detect this radiosource in our Galaxy, in this case Cassiopeia Cas a, at a distance of 11000 light years away from us. Real weak signal.
And that with a small RTLSDR V3 dongle as a receiver.
Of course we had the help of that large 25 meter telescope, gain 55 db, and a LNA especially for 23 cm, build by G4DDK. But still. We are, as CAMRAS, in a privileged position for using that mighty telescope antenna!
Below some pictures (own pictures) of the line up of the observation in the observation room of the radio telescope and the plotter screen with the maximum top of the signal strength.
Pictures above (own picture): in the first picture just visible in the mid the RTL SDR connected to the antenna panel. At the panel just below, we see the HF inputs of the used CAMRAS home SDR, an Ettus B210. The black box just below the control console computer of the telescope.
Picture above: left the SDR # screen, at the right the SpecLab screen with plotter. In the plotter the maximum level top of the observed radiosource. 11000 light years away!
Yesterday I was again at the radiotelescope of Dwingeloo. We had a EME SSTV session there, which was most pleasant.
But I made also a new continuum observation of Cas a with the telescope. The former observation was not satisfactory for me. That could be better, I thought. I made some new adjustments. Adjustments in the plotterprogram of SpecLab and I switched off the AGC in the RTL SDR.
The effect was amazing. Much larger signal of Cas a. In the picture below a screenshot of the plotter screen. You see 2 peaks in it. That’s because I moved the telescope twice in azimut at Cas a.
So was very satisfied now about this result :).
