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2. Basics

We will now talk about some basic concepts that are needed to understand emission from astronomical sources. We will also cover some radio astronomy fundamentals and terms that are needed to understand the language radio astronomers speak.

2.1 Wave Basics

Electromagnetic Radiation

Electromagnetic radiation comes from the acceleration of charged particles. A stationary charged particle has a field associated with it. We cannot see a field but can infer it from the reaction of other particles around the charged particle (for example - the action of a magnet on iron filings). As you move away from the particle the field goes down in strength. Now, if you move the charged particle, this field moves with it and changes. This change in the field causes some energy to be carried away from the particle. It is this energy that we call electromagnetic radiation.

Electromagnetic Spectrum

Wavelength diagramElectromagnetic radiation is characterized by a wavelength. This concept is easily understood if we think about the radiation being in the form of a wave.

The wavelength is defined as the distance between two peaks or two troughs of the wave. The wavelength is inversely proportional to the frequency - which essentially defines how many repetitions of the wave there are in a given intervals. So, as the wavelength increases, the frequency decreases and vice versa.

The different "types" of electromagnetic radiation are defined by their wavelengths:

Type
Wavelength Range
Gamm rays
<0.01 nanometers
X-rays
0.01 - 10 nanometers
Ultraviolet
10 - 300 nanometers
Visible
0.3 - 0.8 micrometers
Infrared
1 - 1000 micrometers
Radio
0.001 - 30 meters

These names are largely historical in origin and are often separated by the techniques used to detect the particular kind of radiation and by the transmission characteristics of the earths atmosphere.

The "radio" regime of the spectrum is further subdivided into several categories that are wavelength dependent.

"Classical radio" wavelengths: The first astronomical detection was made at a wavelength of 14.6 meters. These long wavelengths are what people usually think of as radio wavelengths. The wavelengths at which FM and AM radio and TV signals are sent are also around these wavelengths:

FM radio (and TV): ~ 3 meters

AM radio : 300 meters

Microwaves: These are wavelengths between 1cm and 30 meters. Many of the communication links around the country are at these wavelengths (around 15 cm). Microwave ovens also cook food with radiation at these wavelengths. The important radio emission from neutral atomic hydrogen occurs at a wavelength of 21 cm.

Millimeter waves: Wavelengths from 1 mm to 10 mm. The rotational spectra of molecules occur in this wavelength range.

Submillimeter waves: Wavelengths < 1mm. Radio detection techniques are used to wavelengths of about 0.4mm. This wavelength range overlaps with the far-infrared portion of the electromagnetic spectrum.

2.2 Astronomy Basics

The basics of all astronomical observing is essentially the same. One needs to know where the source is in the sky and how to best use the instrument to get the information required to solve the problem. In general, it is important to know the source position to know whether it is visible at a particular time of day and whether it is in an optimum position for observations. The instrument parameters are different for each telescope although there are some general rules in determining the characteristics of the emission. This will be discussed in the section on calibration.

Coordinate systems - In order to know the source position in the sky one needs to define a coordinate system. Astronomers use the right ascension and declination system which can be compared to the latitude and longitude system on the earth's surface. This system is defined on the "celestial sphere" which is the two dimensional projection of the sky on the sphere around the earth. In this system the zero point of the declination is the "celestial equator" which is parallel to the earth's equator. So, in other words, if you were standing at a spot on the equator you would "see" the celestial equator as an arc passing directly overhead. The declination is 0degrees for a source on the celestial equator and is 90degrees for a source at the north pole. Sources below the equator would have negative declinations. The zero point of the right ascension is the vernal equinox which is the point at which the sun moves into the northern celestial sphere and marks the position of the sun on the first day of spring. The right ascension increases to the east and is measured in units of time.

The next concept we want to define is that of an hour angle. For this, we first define the hour circle as a great circle on the celestial sphere that passes through the north and south celestial poles (which are directly above the north and south geographic poles of the earth). Then, the angle measured westward along the celestial equator from the local meridian to the hour circle that passes through the source is defined as the hour angle of the source.

While making an observation at a telescope we not only want to know the right ascension and declination of the source but also the elevation and azimuth. These parameters will depend on the latitude and longitude of the observatory. If you go outside at night and look for the Pole star (which is situated very close to the north celestial pole), you will see that it is definitely not overhead. The height (in angular units) of the Pole star above your horizon corresponds to the latitude of the place you are standing. The angular distance above (or below) the horizon of any celestial source is called the elevation (or altitude) of the source. The azimuth is defined as the angle along the celestial horizon, measured eastward from the north, to the intersection of the horizon with the great circle passing through the source.

More detailed explanations of these coordinate systems can be found in any basic astronomy textbook.

Time - There are several times that can be defined using the celestial sources. We will only define those that are used actively in observing at the telescope. Universal Time is defined as the local time (defined by the sun) at 0degrees longitude. This is also called Greenwich Mean Time. The local sidereal time is defined as the hour angle of the vernal equinox. From the definition of the hour angle given above we can see that the local time (NOT the standard or zone time) and the local sidereal time will be equal on the day of the vernal equinox.

Doppler motion - This phenomenon describes the effect on the frequency of emission from a source when the source is moving relative to an observer. As the source moves away the wave emitted by the source gets stretched out - this causes the wavelength to increase and the frequency to decrease. It is usally called a "redshift". On the other hand, if the source is moving toward the observer the wave gets compressed causing the wavelength to decrease and the frequency to increase. This is called a "blueshift". The frequency can then be associated with the velocity of the source using the following relationship:

(change in frequency)/(original frequency) = velocity/speed of light

Radio astronomers often use frequency and velocity interchangeably.

Source velocity - The velocity of a given source is an important number in radio astronomy - especially when looking at spectral line emission from molecules. This velocity is defined as a relative velocity - relative to the motion of the "local standard of rest (or LSR)". The LSR is the velocity of the sun and the local group of stars in the Galaxy. So, depending on where the source you are looking at is in the Galaxy relative to the sun, it will either be moving toward the sun or away from the sun. Such a relative motion causes a Doppler effect on the light (or radio waves) coming into the telescope. This can cause the frequency of the emission (or absorption) line that you are looking at to shift. If the shift is greater than your observing frequency window (or bandwidth) then you might miss seeing it altogether. So we want to be sure that the receiving equipment can correctly take this frequency shift into account.

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