Author(s): Tammo Jan Dijkema and Ard Hartsuijker; Translation: Ilse Harmers
Live stream: CAMRAS; Reportage; RTV Drenthe
During the KNVWS National Sun Watching Day July 5, 2020, live streaming from the radio telescope was performed twice, while the radio telescope was pointed at the Sun and explanations were given about the radio telescope and the radio waves emitted by the Sun. Each demonstration lasted about half an hour.
Due to the Covid-19 measures, the radio telescope was closed to visitors. That is why CAMRAS volunteers Tammo Jan Dijkema and Ard Hartsuijker provided this online demonstration.
The Sun is the closest star to Earth and it is fascinating to view in visible light. This can be done safely with special filters and telescopes. Without these filters, the intensity of the Sun is extremely harmful to your eyes. Amateur astronomers and observatories have the right equipment to observe the Sun safely.
The Sun also produces waves outside of visible light, such as infrared (heat), ultraviolet (tones or burns the skin) and radio waves. The special thing about radio waves is that clouds and rain do not block the radio waves from the Sun. The radio telescope is a wonderful instrument to receive the radio waves from the Sun and demonstrate it in a live stream. The wind seemed to become a bummer in the morning with a strong gust of wind – which turned off the engines – but remained calm after that gust.
More than 900 viewers took part in one of the two live streams. After that, the live streams were viewed at least 8000 times on July 5 and 6.
Of the two live streams – about 35 minutes each – Tammo Jan made the 29-minute version below, putting the highlights together and leaving out some imperfections. (Dutch spoken)
RTV Drenthe made the following 2-minute news item of this activity. (Dutch spoken)
When viewing the live streams on YouTube, the chat was used to ask questions. These were often answered by other viewers because Tammo Jan and Ard were the only ones in the telescope and could not continuously monitor the chat.
Chat questions & answers
Click on a question in the fold menu below to read the corresponding answer.
Q1: The Sun is closer to Earth on January 4 than in July. Are the summers in the Southern Hemisphere then warmer than in the Northern Hemisphere?
A1: This difference results from the Earth’s orbit around the Sun being an elongated circle (ellipse). So you would think the summers in the Southern Hemisphere are warmer. But there are many other factors that play a role in this, such as the amount of land and water in the Northern and Southern Hemispheres. The variation in distance of about 5 million kilometers has only very limited influence. However, changes in the elliptical shape can have long-term consequences. For example, read the article What Causes the Seasons? on Spaceplace.nasa.gov.
Q2: What does the abbreviation nm mean?
A2: Nanometer (abbreviation: nm) is one billionth of a meter. Visible light has a wavelength between about 400 nm (violet) and 750 nm (red).
Q3: Are radio waves measured with distance?
A3: Wavelength is a distance. The wavelength is the distance between two consecutive wave crests. Just think of waves one after another in water.
Q4: And what about the frequency? What does frequency of light or radio waves mean?
A4: Here, frequency means the number of vibrations per second. That’s the number of wave crests you see passing in a second. Just think again of waves one after another in water.
Q5: Is the Sun a sphere or a flat surface?
A5: The Sun is a large sphere with a diameter of 1.4 million kilometers (109 Earths side by side).
Q6: How is the speed of light determined? And what does that mean exactly?
A6: The idea that the speed of light is not infinitely large is already very old and will probably come from the fact that the speed of sound is also not infinitely large. You can determine this yourself with regards to sound, for example with a sound echo. The first astronomical measurement of the speed of light was made by Rømer in 1676 using the planet Jupiter and the moon Io. He found a value of 225,000 kilometers per second for the speed of light. In the second half of the 19th century, more accurate measurements were made by Fizeau, Foucault and Michelson. See, for example, ‘How is the speed of light measured?’ on The Original Usenet Physics FAQ. Rounded, the speed of light in vacuum is 300,000 kilometers per second.
Q7: Why is the Sun’s signal strength greater when the telescope is pointing right next to the Sun than when it is directly facing the Sun (because of that dip in the middle)? Is the solar atmosphere (corona) stronger in radio intensity than the Sun itself?
A7: At wavelengths of an inch or less, the radio Sun is about the same size as the Sun in visible light, and the intensity of the radio waves is evenly distributed across the solar disk. At those wavelengths, the radio waves mainly come from the photosphere – the bottom layer of the solar atmosphere – just like most visible light. At decimeter wavelengths (CAMRAS observed the Sun at 23 centimeters), the radio Sun is somewhat larger than the Sun in visible light and we also see that the intensity of the radio waves at the edges of the Sun increases (limb brightening). These radio waves come from the chromosphere – a higher layer in the solar atmosphere. There are more free electrons there than in the photosphere. The radio waves are created by the movement of free electrons in the magnetic field of the Sun. At larger wavelengths, the effect of limb brightening increases. But for radio waves with wavelengths greater than meters, the limb brightening disappears and the intensity of the radio waves in the center is highest. At those wavelengths, the radio Sun is much larger than the Sun in visible light. Those radio waves originate in the corona of the Sun. Because the Dwingeloo Radio Telescope cannot see details smaller than half a degree at a 23 centimeter wavelength, the effect of the limb brightening is smeared and smoothed.
Q8: Why is there to the left of the Sun’s graph a dip lower than the background, and a peak on the right?
A8: The Sun has bright spots here and there and radio waves also emanate from the corona. Antennas such as the radio telescope are very sensitive in one direction (the main beam that coincides with the axis of the parabolic mirror), but they also pick up radio waves from the far less sensitive side beams. Radio waves transmitted to the antenna via the side beams affect the image. In real observations, the astronomer has to correct for this and that means more measuring work than the observation we made in this demonstration.
Q9: What can be learned from radio signals?
A9: Just as astronomers can understand how the Sun or a star is constructed from different color observations of visible light, astronomers can also understand a lot with other parts of the electromagnetic spectrum, such as radio waves. Radio waves in the universe mainly come from free electrons that move in magnetic fields. From this data, combined with other observations, astronomers can much better understand how the Sun is constructed, or the remains of an exploded star (supernova) or a galaxy. But the radio waves of the neutral hydrogen gas in the Milky Way (the 21-cm hydrogen line) and other galaxies also provide information about, for example, how the galaxy rotates and how much mass there is in the galaxy.
Q10: What exactly does it mean: Cassiopeia A is the brightest radio source in the sky?
A10: Sirius is the brightest star in the sky. Of the thousands of radio sources that astronomers can observe in the sky, there is always one that is the brightest: that is Cassiopeia A. Cassiopeia A is not a star, but the remainder of an exploded star. It is about 11,000 light-years away. Incidentally, Cassiopeia A loses the status of brightest object in the sky with very long radio waves and very short ones.
Q11: How long ago did the star that formed Cassiopeia A explode?
A11: From the change in images in visible light of the strings of gas, it has been calculated that this supernova had to take place around 1667. Note: astronomers look back in time. It takes 11,000 years for the light and radio waves to arrive here. So the supernova occurred 11,000 years earlier.
Q12: Do you ever suffer from disturbances caused by, for example, ether pirates or radar installations?
A12: Radio telescopes are extremely sensitive. The radio signals from the universe are extremely weak (which are measured in a unit – the Jansky – which has 26 zeros after the decimal point; Wikipedia). The wavelengths at which astronomers measure are internationally protected; those wavelengths should not be used for other purposes. Radar signals, electric fences, TV and radio masts, telecommunication masts, weather balloons, ether pirates, mobile phones, cars, mopeds, microwaves, etc., but also thunderstorms, it can all cause interference. With good technology, the interference at the interference source can be prevented. The technicians that astronomers work with are very good at filtering out interference using electronics and software, but there is a limit to the possibilities if the interference is too strong or too broadband and it always comes at the expense of the sensitivity of the radio telescope.
Q13: Nice that you use Stellarium. Is the Milky Way that gray area?
A13: Stellarium is a free open source planetarium program for your computer. It is a nice program that shows what you can see in the sky with the naked eye, binoculars or a telescope. CAMRAS volunteers often use it in demonstrations for visitors. That gray area is indeed our Milky Way.
Q14: Is it possible to have the telescope pass by Comet NeoWISE? It is close to the Sun now.
A14: Unfortunately, no. We skip C/2020 F3 NeoWISE. Astronomers have made radio observations on comets, including in Dwingeloo. The radio waves emanating from comets are weak and characteristic of the molecules in the comet. This requires adjustments of the receiver system. The antenna and receiver system with which CAMRAS is currently observing must then be further adapted. That the comet is close to the Sun is not an argument because the radio telescope can reach all directions in the sky. More info about comets on Solarsystem.nasa.gov.
Q15: The Sun has an 11 year cycle. I understood that the peak of that cycle should have been there already. Is it there now or is the peak still to come?
A15: The solar activity is currently at a minimum from which we seem to crawl up. The last cycle was relatively uneventful and it is known that the cycles are not exactly 11 years, but may be slightly longer or shorter and may be very different in activity. For example, read Solar Cycle on Wikipedia and recent info with charts on Solar Cycle progress on Spaceweatherlive.com.
Q16: What was meant by being caught by the wind?
A16: That the force of the wind was stronger than the motor. The motor then stops and everything has to be restarted. The dish is made of mesh; you might think that wind can go through it. But at the mesh size of 7 millimeter, for wind the dish begins to acquire properties of a closed surface. The surface of the dish is 500 square meters. Upon warnings of expected wind gusts in Drenthe of 80 kilometers per hour, the telescope is put in the so-called storm position (pointing upwards) and the four wheels are clamped to the rail track.
Q17: The electric motor looks like an electric motor from grandmother’s time. And do you not have an "auto pilot"?
A17a: Grandmother’s time is right, the entire radio telescope (opened in 1956) is from grandmother’s time. The large motor (painted green in the images) is the original Heemaf motor that can quickly rotate the telescope in horizontal direction (azimuth). That motor is no longer used. For slow motion such as tracking an object in the sky, there were other motors in the telescope. The current motor (also painted green) is much smaller and can be used for both fast spinning and slow movements like tracking. For the vertical movement (elevation) there are comparable motors at the top of the radio telescope.
A17b: We do not have the “auto pilot” as you can buy it from optical telescope suppliers. Until the early 1970s there was a mechanical pilot in the radio telescope that constantly converted the position of an object in the sky (in celestial coordinates of right ascension and declination) with the sidereal time in Dwingeloo to azimuth (angle along the horizon) and elevation (angle above horizon). Now that is done with a computer and special software, so also a kind of “auto pilot”. For example, read Motion of the Stars in Understanding Astronomy (physics.weber.edu/schroeder/ua/) and Celestial Coordinate System on Wikipedia.
Q18: The webcams do not show a clear image.
A18: We have different cameras. Brought from home and indeed not all very new.
Until we remove them, the original live streams can still be viewed at: – first session: https://www.youtube.com/watch?v=2aX-c1PdSCA – second session: https://www.youtube.com/watch?v=4JkerJHFYCA
