Very Simple, Simple, and Other Approaches

Joachim Köppen Strasbourg 2008

Simple radio telescopes on 12 GHz have been described by:

• Little Bitty Telescope is a radio telescope also operating in the Ku band. It is less sophisticated than the ESA-Dresden instrument, but the working principle is the same!
• Itty Bitty Radio Telescope is an even smaller version of the Little Bitty.

• Amateur radio telescope by Leon Ternoy works similarly to the ESA-Dresden instrument, and is quite identical to the smaller dish I have been using to observe the Sun in May 2006. This article (in French) describes a bit more the electronics and the interior of the LNB.

• Article by D.Cleary JRASC 93, 40 (1999) which describes the operation of a telescope just like the ESA-Dresden instrument.
• The Classroom radio telescope by M.Capitolo and W.Lonc is also a simple but effective concept which can be realized with a small budget. It comprises the construction of a simple receiver which overcomes the problems due to the nonlinear indication of the SatFinders!
• The Cyclops@home Solar Radio Telescope by N.Dupont-Bloch features a low-budget computer interface with an ubiquitous analogue-digital converter chip.

• RAMEAU is a group of 10 radio telescopes with 80 cm diameter parabolic TV dishes, operated by Hervé Dole and Laurent Verstraete for teaching astrophysics at the University Paris Sud 11. They use modified SatFinders followed by an 8 bit ADC. Pointing of the antennas is done with Yeasu G-5500 rotators and controllers, as in the ESA-Dresden telescope.

In the manner shown above, I took several drift scan measurements of the Sun, reading the SatFinder instrument at regular time intervals from maximum signal until it faded down to zero. One of these observation is interpreted below: I assume that the SatFinder readings are truely linear to the power of the signal (which is NOT true ...) and fit a simple theoretical pattern for the antenna (the red curve). With this measurement of the width of the antenna beam and a calibration reading taken (or rather estimated) by pointing the dish to the ground, I did obtain a reasonable number for the photospheric temperature of the Sun. One major problem is that the calibration reading is substantially higher than that for the Sun, so I had to make one measurement of the ground and the maximum of the Sun, but another to fully get the dropping of the Sun's signal with time, with two different settings of the 'sensitivity' control!

Below is the Java applet with the data and interpretations of all the solar observations taken with this very simple instrumentation:

Here is a solar scan with the same small dish, the SatFinder as an indicator to find the Sun, but followed by a measurement receiver (from the ESA-Dresden telescope) inside the observatory room, along with the computer. For the flux calibration, the entire telescope was pointed towards the ground, and thus the antenna temperature comes out to be 180 K; the antenna lobe width (HPBW) is 2.8° and hence the temperature of the sun is found to be 6000 K. However, this is lower than we measure with the ESA-Dresden telescope: Most probably, the Sun did not pass right through the centre of the antenna beam!

We can do a bit better: the SatFinder is put in a suitable box together with the power supply and a larger instrument for better visibility during demonstrations. More importantly, the little potentiometer for sensitivity control is replaced by a ten-turn potentiometer, so that any setting can be accurately reproduced and read off. The apparatus can be calibrated by input at 1GHz from a signal generator. This works quite well, but of course we do prefer to use the ESA-Dresden telescope ...

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last update: Apr. 2013 J.Köppen