A Radio Astronomy lab component for university courses

Kedar Soni (MSc. Radio Astronomy), Director, Abhinav Vidyalay, Dombivli, email: director@abhinav.ac.in

Ms. Pooja Bilimogga, MSc Part I (2014), Mumbai University 

Non-visual astronomy employs certain common techniques across all wavelengths, which prove to be an asset for a physics graduate. Besides, these fields have great prospects for wannabe physicists / astronomers. Hence, a constructive, hands-on learning module for graduates and undergraduates in radio physics & astronomy is required.

I have drafted here a brief about radio experiments that could be included into the curriculum at university level, at different levels and as either a project for a longer duration or a regular experiment within a session. The background necessary, will depend upon the specific implementation and the level at which it is being introduced. Details of this are presently out of the scope of this document. Once the level is decided the corresponding theory courses/sessions can be worked out. The infrastructure used at most levels will however remain the same.

• The Concept

• The Experiments (Focus on: Physics, Astronomy, Instrumentation)

• The Instrument (Specifications, Results)

The concept

The simplest experiential learning in modern astronomy begins with an introduction to EMR beyond the visual band and the means to its measurement. This can be accomplished by radiometry experiments in the lab. To work with the radiometer, one needs to understand a variety of concepts like RFI (radio frequency interference), equivalent resistor temperature, wave propagation in conductors and signal formation in the antenna. Along with the basic physics, one also needs some idea of how a receiver works; the various stages and their effect on the signal; also the backend, which could be software for ease of working.

For measuring the radio signal from a noise source, one needs to also know the beam profile (also called the Point Spread Function) and the gain of the antenna. When one moves from the laboratory to the sky, things can get slightly complicated as the interference from variety of terrestrial sources can cause unexplained fluctuations in the signal. Given this noise, it is often tricky to judge where the signal begins and ends. (A prerequisite to calculations) Also the objects in the sky have their motions and one needs to patiently wait for the proper time to do the observations. Hence, a background in lab-based radiometry helps the experimenter work better with the celestial objects.

As far as the instrument itself is concerned, if one is willing to invest in space, technicians and other resources, one can always invest in a full-fledged student telescope with a receiver made for astronomy purposes. However, what I offer here is an amateur system, which can be easily assembled and operated (even repaired), by any undergraduate or non-specialized faculty.

The experiments

Now we could go about three ways of organizing the focus of our experiential learning. The focus on radio physics helps clear basic concepts of under graduates in a regular 2-3 hour lab session and prepares the student for a further course. The focus on radio astronomy is good for a student (late – undergrad or early graduate) who is initiated into astronomy and knows a few of the related concepts. (or is being taught the same as a systematic course) These experiments would usually require an open slot, although the most basic of them (total power) can be performed in a 3-hour time slot. The last area focuses on learning the instrumentation of radio astronomy is more detail and is tailored to give open-ended projects for motivated and advanced graduates. In each category, the experiments are arranged in the increasing order of complexity.

Focus on radio physics

Arrange a basic radiometer to study signal reception and measurement using a noise source. These experiments can be done with or without calibration of the instrument, which is often a little difficult given the fact that the lab has many radio noise sources. Avoiding calibration allows for saving time and simplifies the job of the student.

1.      Verify Lambert’s law – Place a noise source at various distances from the antenna of the radiometer to observe corresponding power in arbitrary radiometer units. The readings can be then seen to follow the inverse-square law of propagation of light. This also helps familiarize the novice with the operations of the radiometer.

2.      Calibrate the radiometer – The radiometer converts the EMR from the source into voltages in various stages including the antenna, the receiver and the digitizer, At each stage there are gains and losses of signals. A thorough understanding of this takes one deeper into communication systems. One can however avoid the hassle and calibrate the system with few measurements using a reference resistor. (Hot and Cold Loads) This technique is further useful while measuring the brightness temperature of the celestial objects.

3.       Get beam profile – Unlike basic optical telescope observations, the radio systems are complicated by the fact that the antenna receives the signals from various angles and in various frequencies. A profile of the beam thus formed when the source is places at different angles with the axis of the antenna, gives the point spread function in the time domain. This later helps analyze the structure in the observations of celestial objects. It is possible to observe the accurate beam profile in frequency domain only with a rather costly spectral analyzer, which can be a later upgrade.

Focus on radio astronomy

There are a variety of ways the celestial sources can be observed depending upon the instrument available. Tracking a source in the sky is most useful for accurate power measurements or for observing pulsars among other interesting things. However, that requires a sturdily mounted antenna with a precision guidance system and compatible motors. This in turn needs regular maintenance and training for the operators. An alternative method is to scan the source as it moves along the sky; the easiest being the drift scan where the antenna is pointed at a future position of the object and the object passes by thus causing a rise and fall of the signal. This can however take long time as the beams of the amateur telescopes are usually very large. Thus, a point-and-shoot technique is best suited for the time-frames of the experiments. These need the student to be adept at pointing the antenna to the positions read from the mapping software.

1.      Sun’s brightness temperature – First calibrate the radio meter for the site and time of the day using the reference loads (Hot and Cold). Now point the antenna directly towards the Sun and let the Sun pass by the beam. This will give a semi-Gaussian. Note the difference caused by the Sun between the highest signal (Sun in beam-center) and the cold sky. (lowest point of the signal after a few minutes) Use the calibration equation to determine the temperature caused by the Sun (and some other terrestrial and celestial sources) at the antenna. Using knowledge of antenna beam angle and Sun’s angular size, determine Sun’s brightness temperature. The other sources around can be assumed to be negligible for a short experiment or they can be studied periodically for a more advanced job. The calibration and three readings take a total of about an hour.

2.      Brightness temperature of bright objects – The method described above for the Sun can be easily applied for other bright objects which may be visible from the location at an appropriate time, such as Virgo A, Sagittarius A, Cygnus A, Cassiopeia A, Taurus A and also the belt of the galaxy. With a bit of skill and reasonably low RFI on the site, even dimmer objects can be measured. The measurement of one object after calibration should take about an hour. The extra skill required here will be the need to point the antenna to an invisible object. This is readily done using a compass and a clinometer. (angle finder) An upgrade would be to use a computerized stand such as the NEQ 6 to move the antenna; however, this apart from the cost would be a hurdle to developing the skills of the student.

3.      Angular size of the Sun or any bright object – The object in the sky if small compared to the beam (point source) moves across the beam simply at the rate of motion of the Earth, i.e. a degree in 4 minutes. This causes the signal to rise and fall to the background value in this time. If the source is however extended like the Sun or Moon, it will traverse for an extra time that corresponds to its size. For this the angular speed of the source (i.e. its rate of drift due to its declination) should be known.

4.      Position of an object (e.g. Sun) – If the telescope pointing is known to a fair accuracy, one can point it near where a source would pass by (deliberately choosing a lower altitude). The time the source displays maximum power, i.e. the center of the signal corresponds with the right ascension of the source. Now if this is repeated by choosing a higher altitude each time while taking into account that the source is moving along the sky, we get multiple scans. By stacking the scans together (or otherwise) we can determine the altitude of the telescope at which the signal was the highest This would give the declination of the source. Here of course, a point-and-shoot technique would not work and a brief drift scan would be needed.

5.      Study of atmospheric absorption / sky background – The atmosphere absorbs radio waves (although feebly at L-band) and attenuates the signals. Students can observe a standard calibrator source like 3C273 typically near the meridian. If the losses / gains of the receiver and the antenna are reasonably known, with a large no of readings, the loss due to the atmosphere can be computed. This loss is especially high during monsoon. Likewise, the sky background can be studied at different angles from the location of the telescope by observing a cold patch of the sky as it passes across the sky, at intervals of about an hour. Either of these experiments are long duration and would require a week or so.

Focus on instrumentation

The experiments above can mostly be done with enough retrials within about 2-3 hours. Certain experiments however require much longer time, many iterations as also stages in which the groundwork needs to be done. Such can be ideal projects for the motivated senior students. These projects can also help the juniors in doing their experiments better as some of the effects of the environment of the system which are otherwise neglected at a beginner stage, can be determined to a fair accuracy.

1.      Making the Antenna – Students can gain first hand understanding of wave propagation and antenna operations by making a variety of antennas from scratch. (use of technicians and a workshop is not essential) One can start from the simplest idea of a can antenna to making a quarter-wave wire dipole and on to a helix or even using a ready-made feed for testing. The antennae made to work within the capability of our receiver can be operating in the UHF and L-band with bandwidths as high as few GHz to as narrow as about 100 MHz. Students can either make one antenna to study its beam, gain and bandwidth in detail or make multiple antennae to study how the parameters varies with design.

2.      Study of the receiver – The receiver being used is a complicated system and however accurate, it is prone to variations and degradations. Also the transmission lines and the other connecting wires can add losses and noise to the signal. So leaving the antenna out and pointing the receiver to standard reference load (termination) one can determine its temperature and noise figure and by observing over a long period (say keep the system running for a day) any variations in the temperature. Then using a standard noise source (Noise diode or a simple CFL kept at a known distance from the antenna) one can determine the net gain of the system. Now by altering different components like the connecting wires (different makes) or sound cards (different PC’s) one can determine the configuration that gives the best results. One can also study the effect of a Faraday cage on the receiver noise. Some assistance from a telecommunications faculty is helpful. The project can be done in a span of 2-3 weeks.

3.      Making a noise source – A very useful device is a noise source either added directly to the receiver or fed into the antenna. Depending upon the frequency of operation and the type of antenna (i.e. far-field, etc.) the source can be constructed using either a simple resistor attached to another antenna or a an electronic circuit. This helps in the calibration and the observations further on.

4.      Study of RFI on site – The radio frequency interference at the location of the telescope can contribute to a large error in the measurements, especially if it is sporadic and unpredictable. There are various contributors to RFI – fans, tube lights, CFL’s, AC’s, power transmission lines, mobile towers, mobile phones, Wi-Fi dongles, etc. One can choose a location where these sources are far away and few in number. However, a long-term study of the patterns in which these noise contributions change on a daily basis can help subtract this background power from the data measured by other regular experiments.

The Instrument

The Specifications

The telescope proposed here has the following components:

1.      Antenna – L-Band Helix (Left Circularly Polarized) mounted on a fully steerable Alt-Az mount typically used for optical telescopes. (The picture on the left shows the 1.34 GHz helix dismounted, while on the right hand side is an older 2.7 GHz helix along with the mount)

2.      Receiver – Direct-To-Home TV Set Top Box (STB) of any company (typically works in L band; channel subscription not required); bandwidth ~ 950 – 2150 MHz; gain for astronomical signals ~  40 dB with inactive AGC; (Auto Gain Controller contributes a sudden and drastic gain variation for powerful astronomical signals)

3.      Digitizer – A standard PC sound card (any make should do)

4.      Backend – Software to plot spectrum / time domain signal like Radio SkyPipe, Zelscope, etc; assorted astronomical freeware.

5.      Calibrator – A 75 Ohm resistor made from five 2W solid state resistors connected via a coaxial cable to the receiver; it is dipped in a bottle of water / oil at ambient temperature (cold 30 deg C) or when warmed up (hot ~ 90 deg C)

6.      Detailed Specs

7.      Beam Profile – In spite of bad RFI at the site, we have measured our beam shape to be as follows. (It may actually be smoother)

The advantage of using a STB instead of a regular receiver is that STB’s are robust and easy to purchase / replace, while making a receiver for our frequency of interest and maintaining it requires technical expertise.

The Helical antenna offers an advantage of mobility and easy-pointing along with the option of studying many celestial objects (as most are relatively brighter in L-band), while the Ku-band dish has an excellent gain to observe the Sun.

Some Results of the Helix

Solar flares appear as massive bursts of radio signal. The vertical axis has arbitrary scaling

Cas A (2400 Jansky at 1.4 GHz) observed with some nasty spikes of RFI in the middle of the data.   

Shown below is the Sun measured on a calibrated system. (Vertical axis in Kelvin) The size of the solar disk is measured along with the brightness of the main disk and the chromosphere. The signal is inverted due to high RF background in the area. (i.e. the peaks are actually the low brightness points on the sky)

A reading taken for Cygnus A with a drift scan, which gives the brightness temperature to be about 28 million K as expected. The calculations were done in MathCAD for simplicity. The blue box shows the readings for the calibrator hot and cold loads, while the yellow indicates the system temperature of about 22 K and a gain of ~ 197.  

This should contribute towards a basic course in Radio Astronomy for undergraduate and graduate students.