How can I build a radio telescope

See the sky with different eyes

Radio waves were the first invisible type of radiation used for systematic sky observation. Radio telescopes could be called our "super eyes". They show us an invisible world, an invisible universe.

While optical telescopes mainly show the radiation from hot gases, i.e. from stars, radio telescopes mainly show the radiation from cold gases, i.e. from what is between the stars, the interstellar medium, which mainly consists of extremely thinly distributed hydrogen. The usual visually recognizable stars are invisible in the radio telescope (with the exception of the sun and supernova remnants). However, a radio telescope “sees” a large number of point sources that have nothing to do with the stars that are visible to our eyes. In the early days they were also called “radio stars” without knowing what they actually were. As we know today, you can see the radiation from active black holes, i.e. active galaxy nuclei and quasars, which are by far the most massive individual objects in the universe. Not to be forgotten is the cosmic background radiation, also known as 3 ° K radiation, which, figuratively speaking, represents the “reverberation of the Big Bang”.

Observing the sky with radio waves - this is where most amateur astronomers avoid

Although professional astronomy and astrophysics, and especially radio astronomy, are currently enjoying their heyday, there are only a few amateur astronomers who devote themselves to radio astronomy. You can immerse yourself in a completely new world and see the sky “with different eyes”, so to speak. But this is where a radio amateur “gets lost” rather than an amateur astronomer. Over 99% of amateur astronomers do not have a radio telescope.

Why is that ?

Anyone who is interested in it immediately notices that you can't see through a radio telescope at all. For that reason alone, radio astronomy seems a bit abstract, because you simply cannot see radio waves directly. What is missing, so to speak, is the romanticism of a direct view of the cosmic events.

Radio radiation is true the same as optical light or infrared light, namely an electromagnetic wave, but it has much longer waves that cannot irritate our sensory organs. Therefore, when actually "observing" objects, one is solely dependent on sensitive detection devices. Except maybe a pointer deflection on a measuring instrument or a shaky curve on a monitor, you don't see anything at first. The measured values ​​obtained must somehow be converted into a form that is appealing to our organs of perception. The most suitable forms of representation for our brain are pictures and as "transformer“Nothing is better than a PC.

If you want, you can also hear something, with cosmic radio sources in the vast majority of cases it is just a steady hissing or hissing noise with no discernible variations, like an old VHF radio set on which the tuned station has been turned out a little.

And how do you get pictures there?

Well, it is not as direct, simple and fast as in the optical case and high-resolution images, like with a light-optical telescope, you don't get any - that disappoints many - many also shy away from soldering. From a visual point of view, a radio antenna is basically just a 1-pixel camera, i.e. even for simple pictures you have to scan the celestial objects step by step, which is time-consuming and time-consuming. But there is no other way if you only have a single antenna. On an optical telescope, this would be comparable to measuring the brightness using a photocell.

With the help of a PC this is not a problem. It makes it possible to automate all work processes and it can be turned into a radio mirror "Robotic" radio telescope polish up. The scanning process can be made easy by moving the mirror up and down on the meridian line in the direction of elevation with just one motor. The movement across it (in right ascension) then ensures the earth's rotation very precisely and, above all, automatically. If you have recorded several thousand measured values ​​with it, you can afterwards, again with the PC, e.g. have a color map of the radio intensity displayed.

And spectra?

Radio spectra are much easier to get than radio images. With an SDR receiver and freeware from the Internet (see below) you always automatically get a constantly renewing high-resolution spectrum - that is really impressive.

Despite the fact that we use radio waves all the time, hardly anyone really knows anything about them

Everyone uses radio waves with their cell phones, with WLAN or Bluetooth, with walky-talkies, with the central locking of the car or with radio remote controls in model making up to microwave ovens and, of course, not least for radio and television.

But behind this there is already a serious problem in radio astronomy that weak radio waves from the cosmos can be easily, but often very massive, and nowadays more and more often superimposed and disturbed by stray waves from earthly, man-made sources. The question then arises as to how cosmic radio sources can be distinguished from earthly radio sources at all or how one can separate or block out the effects of earthly sources of interference as far as possible.

Then why do you do it when it is so difficult?

That is the attraction of the matter, to recognize the problems and to get them under control and to find ways to accomplish this without great expense.

The essential thing, however, is the following: After processing the measured values, you get completely different information about the celestial objects, which you couldn't have gotten from light, for example, or what is even more exciting, you can detect and measure objects that have no light at all emit, e.g. the large unionized hydrogen clouds of the interstellar gas (HI clouds).

Radio waves allow us the "big view"

Because of the quantitative dominance of hydrogen in space, it is also the most productive source of information for amateur radio astronomy with its 21cm radio radiation. It even allows us a three-dimensional thinning of our Milky Way with a small radio telescope.

The “ingenious” thing about this radiation is that it is a line radiation, ie radiation at a very sharply defined frequency (1420.4 megahertz). Due to the Doppler effect, very precise speeds can therefore be determined from the frequency shift of the various hydrogen clouds in the Milky Way. But that's not all.

Because radio waves are not stopped by the galactic dust clouds like light, celestial objects can also be detected and measured well through interstellar dust clouds. An important example of this is the center of the Milky Way, where a supermassive black hole with 4.1 million solar masses sits.

Since the Milky Way is a dynamic structure, especially because there are different orbital velocities at different distances from the center of the Milky Way, hydrogen clouds that are one behind the other and therefore overlap for us can also be registered and displayed separately via the Doppler effect. So it is not like in an X-ray image in which different bone shadows can overlap and this makes the overview more difficult.

Furthermore, a corresponding model of the galactic rotation can also be used to localize the hydrogen clouds in depth. In the process, many of the hydrogen clouds that are optically invisible to us come to light, which are hidden behind the galactic dust clouds. However, this requires a large number of measurements and it is also a hard task to evaluate them in the direction of a 3D model of the Milky Way [13]. One of the greatest triumphs of radio astronomy in the 1950s is based on this. In this way it was possible to prove that the Milky Way is a spiral nebula!

Indications of large amounts of "dark matter"

If the whole sky is carefully surveyed, the movement model of the whole Milky Way can also be checked in reverse. This then leads to the seemingly adventurous hypothesis of “dark matter”. Since the spiral arms, especially in the outer parts, rotate unexpectedly quickly around the center of the Milky Way, one does not initially understand why the Milky Way does not fly apart. To explain this, one needs about five times as much dark matter in addition to visible matter, ie matter that is also "invisible" in the radio range, in order to have enough gravity that the Milky Way is held together and does not fly apart.

The phenomenon of rotating too fast also occurs in all other galaxies, but here you can also measure the rotation optically, because you can look at the other galaxies more or less obliquely from the outside and their spiral arms are in most cases not hidden behind clouds of dust are. With radio telescopes it is also possible to follow the spiral arms much further outwards than is optically possible.

What does the starry sky actually look like in the radio area?

Some things are "turned upside down" here

You can't see the usual starry sky in the radio area at all! The only star is just that Sun to be seen because it is simply about 250,000 times closer than any other star. Also the moon makes a clear signal even with a small satellite TV dish.

The most dominant "object" after Sun is funny enough ground. Houses, trees and people also shine brighter than the sky or the moon by day and night. That means, earthly objects must be well shielded or must not stand in the beam path if one wants to detect weaker cosmic signals.

What still stands in the way is that Earth atmosphere. It "glows" in the radio area and makes an increasingly stronger underground towards the horizon. This phenomenon is particularly annoying with small antennas, because they have a very wide “receiving lobe” and thus unfortunately absorb a lot of atmospheric background radiation and weak point sources “drown” in them.

At higher frequencies (from approx. 1 gigahertz) water vapor in the atmosphere causes increasing problems and from around 20 GHz you have to move to the driest deserts and heights above 5000m (e.g. ALMA observatory in Chile). Earthly Clouds therefore make an additional, variable underground (depending on the thickness of the clouds and the observation frequency)

The Hydrogen clouds in the spiral arms of the Milky Way can also be easily detected, even with small satellite TV dishes. However, you need a modified receiver head or LNB (e.g. the can feeder described below) that is tuned to the hydrogen frequency (approx. 1420 MHz). With this it is already possible, as already mentioned, to “see” several spiral arms standing one behind the other at the same time through the galactic dust clouds.

What can still be proven is “synchrotron radiation”. It has completely different causes than the above-mentioned 21cm hydrogen radiation, which is emitted by atoms. Synchrotron radiation occurs when free charged particles move along magnetic field lines on helical spiral paths. The continuous radiation of the Milky Way is an example of this. Also the brightest quasar, called "Cygnus A" and Supernova remnants, such as "Taurus A" mainly show synchrotron radiation in the radio range and are still easily detectable even with smaller radio telescopes.

Cygnus A is an active supermassive black hole in a galaxy core 750 million light years away. Taurus A is the crab nebula M1, but not the crab pulsar, this is much weaker than the nebula. The radio source "Cassiopeia A" is also interesting. It is the brightest extrasolar radio source and, until recently, was the youngest supernova remnant discovered in the Milky Way. It is one of several supernova remnants that were first discovered in the radio range because they are hidden behind galactic clouds of dust and are therefore optically invisible. Even the explosion of "Cassiopeia A" in 1680 could not be optically detected. Nevertheless, the explosion flash can still be examined optically today because the scattered light from the surrounding interstellar dust is only now reaching us.

Also Jupiter is not exactly a quiet fellow when it comes to radio waves.

How can you get started?

Knew how“Is the motto

If you don't want to spend a larger 4- or 5-digit amount right away (radio telescopes are not mass-produced), you can only find happiness by building it yourself. To do this, you have to have some basic knowledge of high-frequency technology in order to get ahead with little investment. What are the options?

TV satellite antennas

The simplest and cheapest are TV satellite antennas (with LNB around 30 €) - they can be used to do exciting experiments without soldering. The frequency range is between 10 and 12 gigahertz. The sun and moon go instantly, even if you only have one "Satfinder“(For about 10 €) as a recipient. It squeaks very loudly when you have aimed well and, for example, finally has the sun "inside". However, the resolution is still modest. Despite the high frequency (10-12 GHz) it is only 2.5 ° - e.g. sunspots cannot be "seen" with it. It is funny that you can use it to “see” the sun through thick clouds or even when it is raining.

It's more difficult with the moon, it's already about 100 times weaker than the sun, but it still comes out clearly. The supernova remnant "Cassiopeia A" can also just be detected. What you can of course “see” are the many TV satellites on the celestial equator, that's what the SatTV dishes are built for after all. However, it is easy to confuse the sun with such a satellite, especially when the sun is close to the celestial equator. The distinction can only be made with a satellite receiver, for example. In the sun, it simply doesn't show a single TV program, despite a strong signal and optimal antenna alignment.

Do we emit radio waves ourselves?

It is astonishing when you stand in front of a satellite dish or when you hold it towards the floor. The registered signal goes to full deflection. - Does that mean that we ourselves or the earth radiate radio waves? - Yes it is ! Some fearful natures will not want to believe that, since even weak radio waves are supposedly dangerous in their view. The fact that all bodies in nature and the environment radiate radio waves without human intervention comes from their own heat. Radio waves are only dangerous when the intensities are as high as in a microwave oven (far more than 1 million times higher than in nature). In all substances that contain water, heat can be produced vigorously and quickly at such intensities.

The tiny amount of radio radiation that you register when you stand in front of a satellite TV dish is the long-wave extension of the heat radiation, the main part of which is at much higher frequencies.

A "can feed"a tin can as a receiving head

A "can feed" is a waveguide built from a larger tin can with an internal monopole antenna - it does amazing things. It is very useful for frequencies in the region of the 21 cm hydrogen line (1-2 GHz). Since an incoming wave is reflected at the bottom of the can, this reflected wave is superimposed on the incident wave to double the wave height and is thus amplified. A small wire antenna inside the can, as long as a quarter of the wavelength, picks up this amplified wave at the point of the strongest overlap. This means that the radio emissions from the interstellar hydrogen clouds in the spiral arms of the Milky Way can already be detected if a good, low-noise preamplifier is installed behind the small wire antenna.


SDR receiver

Another ingenious and also inexpensive component is a so-called SDR receiver (“Software Defined Radio”, from € 20). It can be plugged into the PC like a USB stick and with freeware from the network (e.g. SDR #, HDSDR or GNU radio) you can already hear something, i.e. analog or digital broadcasting. Above all, you automatically see a high-resolution radio spectrum on the computer display.

What is going on there can best be compared to turning the tuning button on an old radio, only the SDR software does this automatically and turns back and forth very quickly, so to speak. In this way he generates a constantly renewing radio spectrum. The PC thus becomes a respectable “spectrum analyzer” at almost no cost. If you connect a sensitive antenna in front of it, e.g. the above-mentioned can feed, you already have an "open source" radio telescope. If you put a TVSat dish in front of it, or if you replace the LNB of the satellite dish with the above-mentioned can feed, the whole thing goes much better (see also my article "From the garden to the galaxy" in the magazine Sterne und Weltraum, September 2017 issue).

Parabolic mirror

For very weak signals you need an antenna that can bundle the finely distributed radio radiation very well. This is best done with a parabolic mirror (looks like an open umbrella, just turned up). A SatTV dish is exactly that. All in all, with a 3-meter parabolic mirror and the downstream electronics (preamplifier and SDR), you get, listen and be amazed, around 30 million times the amplification.

A big advantage of this type of antenna is that you do not have to convert the entire antenna when you change the reception frequency, only the comparatively small reception head.

However, a parabolic mirror has another important advantage: it largely shields the radio radiation from the ground and therefore helps to ensure that the weak cosmic radio sources come out better and are not so strongly covered by interference radiation.

Larger parabolic mirrors up to several meters in size, i.e. significantly larger than satellite dishes, can bestressed ribs"-Principle made from simple hardware store parts, e.g. with PVC cable ducts as ribs and cellar shaft covers made of fine aluminum wire mesh as reflector grids. The tubes are clamped in a star shape at one end and simply bent upwards at the other end. This automatically creates a parabolic shape. However, because of their “jello” properties, such cheap giant mirrors should only be installed permanently. Each rib is firmly connected to the ground and the mirror is then constantly aligned vertically upwards, similar to the well-known 300m Arecibo mirror in Puerto Rico.

Despite the immobile installation, such a mirror can also be used outside of the focal point (in the so-called “off-axis” area) to scan the sky. To do this, you only need to put the above-mentioned can feed downwards over the mirror and swing it around in a plane at a distance from the focal point. In addition to the hydrogen clouds in the Milky Way, a number of weak radio sources such as supernova remnants, active galaxy nuclei and quasars can then be detected. From a size of 6 meters and measuring times lasting hours, the first bright pulsars should be detectable.


If you want to delve deeper into the topic and are skilled, interferometry measurements can be carried out with two SatTV dishes at no great cost and, for example, large areas of activity on the sun can be detected with 3 dishes. In addition to the second or third satellite TV mirror, you also need a video grabber (with a USB connection).

With interferometry, the resolution of the images obtained can be increased almost at will. The data acquisition and the data processing in the computer are then quite demanding and extensive. Suitable free software from the Haystack Observatory is available for basic experiments on interferometry with simple SatTV mirrors.

Brief facts:

  • Radio emissions from space is in 1933 looking for the cause of Radio interference (RFI, see below) was discovered by Karl Jansky at Bell Labs in the USA. That was mainly the so-called "sky noise" that comes from the Milky Way. In specialist astronomy, this discovery was not really taken seriously at first, and a development of this observation technique was actually overslept until the end of the Second World War.
  • As beautiful and large a parabolic mirror antenna is, it is still basically just one 1 pixel camera.
  • one only has 1 antenna, the Airy disk defines the resolution (in radio technology Receiving lobe because it looks like a club when shown as a polar diagram), i.e. you only have a resolution of the size of the Airy disk, you can only do raster scans or record spectra and there is no "zoom" at first (-> table)
  • If you have several antennas, you can "zoom in" on the Airy disk: Interferometry is exactly the "zoom" - but it is complicated.

More facts and details

Sources of interference or RFI ("Radio Frequency Interference")

As already mentioned at the beginning, it is often difficult to distinguish unintentional scattered radiation from electrical devices in the vicinity from the real signals from the cosmos. You have to use a few tricks and gain some experience until you can be sure that you are not catching the stray waves from the USB cable that is currently being used by a neighbor or even the stray radiation from your own PC ... But this type of troubleshooting was the origin of the Radio astronomy, when attempts were made in the 1930s to track down interfering signals in worldwide radio traffic - only here it was the other way round, here the cosmic radio sources were regarded as the sources of interference.

The strongest natural source of interference is the ground, whose radio radiation should not reach the receiving head because of its warmth. Otherwise the weak cosmic signals would "drown" in the radiation swamp of the earth. Due to its size, the parabolic mirror automatically prevents the worst, it largely shields the earth's radiation. The receiving cone of the receiving head, which is directed downwards towards the parabolic mirror, must nevertheless be well aligned with the parabolic mirror and matched to the size of the mirror and the distance from the focal point of the mirror.

Physics of cosmic radio emissions

The radio range is huge compared to the light-optical range - for example 20 octaves. The visible light, the perception range of the human eye, has only 1 octave.

Roughly two cases can be distinguished between the causes of the generation of radio radiation: thermal radiation (due to its own heat) and non-thermal radiation, which cannot be characterized by a temperature.

  • thermal radiation: Earth, houses, trees, people (including a hand in the beam path!), Sun, moon, earth's atmosphere and clouds. To be more precise, thermal radio radiation is the branch of thermal radiation in the radio range, which accordingly shows a continuous spectrum.
  • non-thermal radiation: these include the line radiation of the hydrogen atom or molecules from the Milky Way and the synchrotron radiation from Jupiter, the Milky Way, supernova remnants and active black holes. Synchrotron radiation also shows a continuous spectrum. In all cases, the cause is charged particles moving helically along magnetic field lines.

Another problem is the movement of the earth around the sun. For example, if you observe the spectral lines of hydrogen at a wavelength of 21 cm, the position of the spectra changes tremendously over the course of a year. They are pushed back and forth by the Doppler effect, just like in the optical field. The shift is strongest when one looks in the direction of the earth's movement or exactly in the opposite direction. At right angles to it, the shift disappears. In order to be able to compare spectra from different directions or from different seasons, you have to apply a "heliocentric correction", ie convert it so that the spectra are as if you were looking at them from the sun. Professional astronomers use a more precise construct here, namely the “local standard of rest” (LSR). Here the mean speed of movement of the sun relative to the neighboring stars is still taken into account.

Literature and Links

  1. Leech, M .: A 21cm Radio Telescope for the Cost-Conscious (contains assembly instructions for the receiver head):
    (including interferometer with simple SatTV mirrors)
  3. (VLNA = "very low noise amplifier", low-noise preamplifier "G4DDK")
  4. (VLNA, "SAWbird + H1t")
  5. (SDR receiver "Airspy" with USB, SDR = "software defined radio")
  6. (sdrsharp package with "Astrospy" software and "Zadig" driver)
  7. (mathematics software "Matlab home edition")
  8. J.D. Kraus: Antennas, 2nd ed., McGraw-Hill 1997
  9. T.A. Milligan: Modern antenna design, 2nd ed., Wiley Interscience 2005
  10. Wilson, Rohlfs, Hüttemeister: Tools of Radio Astronomy, 6th ed., Springer 2013
  11. J.Ebersberger: "From the garden to the galaxy", SuW 9-2017, p.64ff
  12. J.Ebersberger: “A deep look into the Milky Way”, SuW 5-2020, p.74ff
  13. T. Lauterbach: Radio Astronomy (essentials)
    Basics, technology and observation possibilities of small radio telescopes, Springer Spectrum, 2020
  14. Reich et al., Astron. & Astrophys. Suppl. Ser 83, 539 (1990): The Effelsberg 21 cm radio continuum survey of the Galactic plane
  15. (21cm panorama images of the Milky Way from the Effelsberg 100m radio mirror or from Stockert, also in the Eifel)
  16. (21cm line profiles from the Argelander Institute)
  17. (frequency scan video around the 21cm line, LAB survey)
  18. (own frequency scan video, recorded with my robotic radio telescope)
  19. (other own radio astronomical "Works of art"As videos)