Based on the wishes of astronomers, the receiver laboratory in Dwingeloo developed the most sensitive and stable measuring receiver systems in-house, despite the fact that the available and suitable components and materials in the early years were sometimes scarce on the market. Receiver construction showed a continuous development towards ever larger and more complex receiver systems that were also cooled with liquid nitrogen later on. This development was also visible in a continuous growth of the staff size.
21 cm hydrogen line receiver
For the first 21 cm hydrogen line receiver developed for the Dwingeloo Radio Telescope, the experience gained with the receiver in Kootwijk came in handy. Radio-receiving equipment of that time consisted of an antenna, waveguides and coaxial cables. The amplification of the radio signal was done with equipment composed of separate components such as radio tubes (which produced a lot of heat), coils, capacitors and resistors.
Stability and sensitivity
Characteristic properties of receiver systems for radio astronomy are stability and sensitivity.
The stability of the equipment was a big problem: the amplification of receivers varied, albeit slowly. In addition, radio tubes themselves produce noise that is many times stronger than the received signal from the hydrogen in the Milky Way.
Receiver sensitivity depends on receiver noise, but also on used bandwidth and observation time. The sensitivity increases with the square root of the observation time. So a longer observation time means better sensitivity or a better signal-to-noise ratio. However, the problem of stability – the slowly varying receiver properties – is getting bigger again. Thus, the receiver must be kept stable throughout the observation time.
Trick with radio background noise from the sky
Radio astronomers could manage stability and sensitivity by comparing their measurements with a constant signal source. For this, they used the radio background radiation of the sky itself at a frequency that differed slightly more than 1 MHz from the frequency of the 21 cm hydrogen line (1420.406 MHz). By quickly switching the receiver back and forth between the frequency of the hydrogen line and the 1 MHz shifted frequency of the sky background, it was always possible to compensate for changes in the gain of the receiver. Switching back and forth happened 400 times per second, with the disadvantage that only half the observation time is used. As a result, that worsened the signal-to-noise ratio, which depends on the root of the observation time.
The second trick was a double receiver: one receiver switched between the frequency of the 21 cm line and a frequency that was 1 MHz higher, the other receiver switched between a frequency 1 MHz lower and the frequency of the 21 cm line. By summing up the signals from both receivers in a smart way, a stable and well-calibrated signal with sufficient signal-to-noise ratio was obtained.
At present, much of the above equipment is miniaturized in compact circuits and chips on printed circuit boards. Several spectral line receivers for 18 and 21 centimeters have been built. The last line receiver for the Dwingeloo Telescope was an accurate 1024-channel (auto)correlator, the radio equivalent of a spectrometer. This correlator could distinguish velocity differences in the hydrogen gas up to 1 km/s. In the early 1990s, with this receiver a last and very detailed study was made of the neutral hydrogen in our Milky Way.
The hydrogen line receivers were also used for research of extragalactic systems. Therefore, sometimes, the receiver had to be tuned to a lower frequency because of the redshift. In 1959, an extraordinary frequency adjustment was made of 1420 MHz to below 1389 MHz for a hydrogen survey in the Coma cluster.
Continuum and polarization receivers
In addition to the spectral line receivers, various receiver types have been built for different wavelengths between 6 and 92 centimeters. Those receivers were used to observe the Milky Way continuum radiation and the polarization thereof.
Typical for a polarization receiver system is a double receiver coupled to a rotating crossed dipole pair at the focal point.
Interferometry and Very Long Baseline Interferometry
In an interferometer setup with one of the two 7,5 meter Würzburg antennas located next to the radio telescope, the 1415 MHz antenna pattern of the radio telescope was measured in 1970.
In 1976, the Dwingeloo Radio Telescope joined at 1612 MHz as an element in a ‘Very Long Baseline Interferometry’ network with the 100-meter telescope in Effelsberg (Germany), the 25-meter telescope in Onsala (Sweden) and a 26-meter telescope in Fort Davis (Texas, USA) as counter stations.
Since 1972, the Dwingeloo Radio Telescope was also used for solar research. The receiver consisted of a 60-channel spectrograph for 160-320 MHz, and later tuned to 400-900 MHz. In order to be able to observe eruptions on the Sun, it was necessary to switch between receiver systems within fifteen minutes. This demanded a radical change with the result that the tripod was replaced by the four leg in 1974.