Lecture - Motivation and Background

Radio telescopes play a tremendous role in modern astrophysics due to the manifold possibilities of observation. With radio astronomy, we are able to observe a broad range of thermal (e.g. H21-line at  a wavelength of $\lambda = 21\,\text{cm}$) and non thermal (e.g. synchrotron-) radiation. Radio astronomy also plays a unique role when investigating the most powerful objects in the universe, such as active galactic nuclei (AGN) or even the cosmic microwave background (CMB).

For information about the history of radio astronomy beyond the scope of this course, we recommend the book "The Invisible Universe" by Gerrit Verschuur (2015, Springer).

In the following a short summary of the history of radio astronomy will be given.

• 1931: The radio engineer Karl Guthe Jansky was assigned to study the electromagnetic radiation coming from thunder storms by the Bell Labs in Hondel, USA. The scientific focus was the polarization and the direction of arrival of the radiation. With a big, rotating, vertically polarized antenna at a wavelength of $\lambda = 14.6\,\text{m}$ $(\nu = 20.5 \,\text{MHz})$ he was the first person to discover electromagnetic waves from space. In subsequent studies, he inferred that this radiation came from the center of the Milky Way.

• 1937: The first radio telescope devoted solely to astronomical research was built (and paid for) by Grote Reber in his backyard. The size and broad receiving band of $160\,\text{MHz}$ to $3.3\,\text{GHz}$ lead to a series of new discoveries:

• The radiation spectrum of the Milky Way cannot be described by Planck's formula for black body radiation. Different, non thermal processes cause this radiation.
• The first radio astronomical sky survey could be performed. Several strong radio sources in constellations like Sagittarius, Cygnus, Cassiopeia, Canis Major, Puppis and Perseus were discovered.

• 1944: Hendrik van de Hulst proposed that interstellar, neutral hydrogen $HI$ may cause radiation in the radio regime.

• 1951: The $HI$ emission at $\lambda = 21 \,\text{cm}$ (H21-line) was discovered by Ewen and Purcell.

• 1965: Arno Penzias and Robert Wilson tried to verify that the sky temperature at the zenith is approximately $0 \, \text{K}$. However, they detected a $3 \, \text{K}$ background radiation at $\lambda = 7.4 \,\text{cm}$. For this first detection of the CMB, which is the remnant of the Big Bang, Penzias  and Wilson shared a Nobel prize with Pyotr Kapiza in 1978.

• 1967: Jocelyn Bell discovered the first Pulsar.

Fig. 1.1 Arno Penzias and Robert Wilson standing at the Holmdel Horn Antenna with which they discovered the CMB. Credit: NASA

Interferometry is the key technique for achieving much higher angular resolution than is possible with single apertures. Since its modest beginnings in the 1950s and 1960s, larger and more complex interferometer systems have been built to map the brightness distribution in the sky. Some focus on imaging small-diameter radio sources, and their angular resolution can far exceed the resolving power of the largest optical telescopes on the ground or in space. In contrast, low-resolution arrays with many baselines can image faint, scattered radio emissions. The most important link between interferometer observations and the brightness distribution of a source is the Fourier transform, which we introduce in chapter 2.

Here is a brief overview of the evolution of synthesis techniques:

• The Michelson stellar interferometer: Between 1890 and 1921, the first measurements using optical interferometer instruments were made by using two separated receiving apertures measuring fringe amplitudes to determine stellar angular sizes.

• The first astronomical observations with a two-element radio interferometer: In 1946 Ryle and Vonberg measured the correlation of radio flux emitted by the sun with the appearance of sunspots.

• The first phase-switching interferometer: In 1952 the first successor to the previously used $\textit{adding interferometer}$, the phase-switching interferometer, was implemented and adopted by multiple arrays during that time. This significantly reduced common issues with the adding interferometer. A more detailed analysis of this will be given in chapter 2.

• Beginning of astronomical calibration: During the 1950s and 1960s optical identifications of known, small-diameter, radio sources allowed for accurate calibration of interferometer baselines and instrumental phases. Single dish radio observations cannot reach the resolution capabilities of optical telescopes, which is why only accurate radio interferometers are capable to cross correlate astrometry with optical data, because here resolution capabilities can be similar.

• Early measurements of angular dimensions: From 1952 onward, variable-baseline interferometers were used more and more often in measurements.

• Use of solar arrays: From the mid-1950s onwards, multiantenna arrays sensitive to centimeter-wavelengths were developed to provide detailed maps of the solar disk.

• Use of arrays of tracking antennas: From around 1960 onward, meter-wavelength arrays with non-tracking antennas became less prominent while centimeter-wavelength arrays with tracking antennas became more common due to improved computation capabilities. As well as that, separate correlators for each baseline were implemented, instead of a central one for all baselines.

• Earth-rotation synthesis: Introduced in 1962 by Ryle and Neville, the first measurements using earth rotation synthesis could be made. The parallel advancements in computer technology during that time were crucial in providing the necessary tools to process the data, especially performing Fourier transforms.

• Spectral line studies: The aforementioned advancements in electronic engineering allowed for spectral line studies with radio interferometer instruments from around 1962.

• Development of image-processing techniques: From around 1974 onward, efforts were made to develop imaging techniques to translate the interferometric visibilities in spatial brightness distributions in a standardized fashion. The first approaches were also based on phase and amplitude closure as well as nonlinear deconvolution.

• Very long baseline Interferometry (VLBI): The first measurements making use of VLBI were made in 1967. After a few years of improvements and successful operation of multiple programs, the first detection of superluminal motion in active galactic nuclei was discovered by Whitney et al. (1971). Christodoulidis et. al (1986) were able to detect contemporary plate motion, and more accurate astrometry was attained by defining and adopting the International Celestial Reference Frame (Feissel and Mignard, 1998).

• Millimeter-wavelength instruments: From the mid-1980s onward, major developments were made in interferometry arrays covering 100 to 300 GHz.

• Orbiting VLBI (OVLBI) / Space VLBI: The first array that included space satellites in the U.S. Tracking and Data Relay Satellite System (TDRSS) experiment took place from 1986 to 1988. Later, in 1997, the first space VLBI dedicated mission was launched by Japan: an 8-meter radio dish, operated by the VLBI Space Observatory Programme (VSOP). In 2011, the Russian 10-meter radio satellite was launched in order to perform VLBI measurements with ground based telescopes by the program called RadioAstron.

Of course, this list is a small sample of the many developments and accomplishments that took place in this field. This is also a list that could be continuously updated. But we want to show only an extremely brief list of important milestones, relevant to this course, to help the reader keep a historic overview of general developments.

Radio interferometers are used to measure the fine angular features of the radio radiation of the sky. The resolution of single radio antennas are restricted to a few tens of arcseconds by physical limitations, which is insufficient for many astronomical purposes. For example, consider the Effelsberg 100 meter radio telescope: the beam-width of the antenna with a diameter of 100 m and a wavelength of 7 mm is about 17 arcseconds. From an engineering standpoint, steerable radio telescopes cannot be built much bigger than that. In contrast, the diffraction limit for large optical telescopes (~8 m in diameter) is about 0.015 arcseconds. However, the angular resolution of optical telescopes achievable from the ground with conventional techniques (i.e. without adaptive optics) is limited to about 0.5 arcseconds due to the turbulence in the troposphere.

Fig. 1.2 The 100-meter radio dish in Effelsberg, Germany. The control room is located to the left of the dish in the picture. on the bottom right a LOFAR station is also visible. Credit: Peter Sondermann, VisKom/City-Luftbilder

It is important to measure the positions of radio sources with sufficient accuracy in order to connect them with objects detected in the optical and other parts of the electromagnetic spectrum. It is also very important to be able to measure parameters such as the intensity, polarization and frequency spectrum with similar angular resolution in both radio and optical regions. Only by using interferometry techniques in the radio regime is it possible to achieve this. Astrometry is the endeavor of accurately measuring the angular positions of stars and other distant objects. This includes the study of the small changes in the positions of the sky due to parallax caused by the Earth's orbital motion, as well as the changes due to the proper motion of the objects. A precise understanding and mapping of objects in space are important to deduce cosmological parameters.

Astrometric measurements are also a way to test the general theory of relativity and to determine the dynamical parameters of the solar system. For astrometric measurements, it is important to establish a reference frame for the celestial positions. A frame based on extremely distant massive objects as position references would be best. Radio measurements from distant, compact, extragalactic sources currently offer the best conditions for establishing such a framework. The radio technology provides an accuracy of 100 micro-arcseconds or less for absolute positions and 10 micro-arcseconds or less for relative positions of objects that are close together at an angle. Optical measurements of constellations seen through the Earth's atmosphere make it possible to determine positions to an accuracy of about 50 milli-arcseconds. However, using the Hipparcos satellite, the positions of $10^5$ stars have been measured to an accuracy of 1 milli-arcseconds. The Gaia1 mission is expected to provide the positions of $10^9$ stars with an accuracy of 10 micro-arcseconds.

As part of the measurement process, astrometric observations involve determining the orientation of the instrument with respect to the celestial reference frame. Ground-based observations therefore provide a measure of the variation in the Earth's orientation parameters. In addition to the well-known precession and nutation of the rotation axis direction, there are irregular displacements of the Earth's axis with respect to the surface. These shifts, called polar motions, are attributed to the gravitational effects of the Sun and Moon on the Earth's equatorial bulge and dynamic effects in the Earth's mantle, crust, oceans, and atmosphere. The same causes lead to changes in the Earth's angular velocity, which are accounted for with corrections in the universal time system. Measurements of the orientation parameters are important for studying the dynamics of the Earth. 

The benefits of using radio interferometry techniques allows for various studies in the following topics:

• Astrometry
• Geodesy
• Studies of Earth's troposphere and ionosphere
• Solar studies
• Solar-Terrestrial relationship
• Cosmology
• Evolution and mechanisms of
• Stars
• Pulsars
• Galaxies (active galactic nuclei, radio galaxies, masers, star forming galaxies, ...)
• Supernova remnants
• Star forming regions

Note: This list is by no means complete and just highlights the number of fields of study that are impacted by radio interferometry techniques.