Projects for Radio Telescopes

This text was taken almost word for word from the Radio Astronomy Teachers' Notebook, available from us. It is thus written with teachers in mind as the audience, however, it seems appropriate for use here. Probably the most common question you will receive from a neighbor staring at your newest antenna project is "What can you do with it?" While it may be tempting to say, "blow up low flying aircraft", it just isn't prudent.

Unlike visual astronomy, the radio variety does little to stimulate the senses in a direct way. It is a challenge to put a small radio telescope to use in a way which is stimulating as well as informative. Still, there are many possible avenues which may be followed in this effort. The specialities of Jupiter and meteor observations are covered elsewhere in this document (yet to come) . A few suggestions for projects using standard radiometer type radio telescopes are presented below.

It is hoped that detailed observing plans will be generated by those teachers who go forward with these or other ideas, and that those same instructors will see fit to share these plans with others in future updates to this notebook.

Solar Periodicity

A broad beamed or sun tracking radio telescope can be used to follow solar activity levels for a period of two to three months. The data is analyzed for periodicity which might relate to the rotational rate of the sun. Note that rotation occurs non-uniformly with regards to latitude. One group of students could be responsible for visual tracking of sunspot groups and an effort could be made to correlate these observations with the radio data.

The Search for HEPs

High Energy Pulses (HEPs) have been reported by amateur radio astronomers for a number of years. Some of these pulses are associated (at least seemingly) with the region of the sky corresponding to the center of our Milky Way galaxy. Recent observations of gamma ray bursts from this region by professional astronomers would seem to lend some hope that these amateur observations may have some basis in fact. Confirmation that HEPs are of non-terrestrial origin must rely on simultaneous observations by widely separated observers. This requires a coordinated effort, one that might be achieved by two or more classes working in concert. Even if such a search achieved null results, there could be much learning in the process. A method for sharing and correlating observations would be needed. Communication by modem and computer would be a possibility. Even without cooperation between amateurs, the simultaneous observation of an HEP radio event with a professional observation of a gamma ray burst would be of significance. Highly accurate timing calibration would be needed. An instrument of typical amateur beam width should perform the task quite nicely. Altitude and azimuth tracking would increase the amount of time which the galactic center could be observed.

Identification of the Hydrogen Emission Line

It is quite possible to demonstrate the existence of the emission of energy by interstellar Hydrogen clouds. A spectral display can be easily produced using an oscilloscope, ramp voltage generator, and a downconverter stage which is tuned by the application of an external voltage. Such a unit is described in a separate section on this notebook (but not in this web document). The presence of interstellar hydrogen is observed as a bump in the oscilloscope display. The resolution will depend upon the bandwidth of the receiver as well as the sensitivity of the unit. A narrow bandwidth will produce a sharper hydrogen line image but will suffer somewhat in sensitivity. If a good steady frequency standard is available for calibration, there is even the possibility that the doppler shift of the line can be observed. This suggests the possibility for mapping of the relative motions of different areas of the sky. While such a project might seem ambitious, it would seem to be within the reach of many.

Mapping the Plane of the Milky Way

A simple project, within the reach of very modest instruments would be the discerning of the generally flat shape of the Milky Way. The project could be undertaken using simple drift scan techniques. As the angular distance of the observation from the galactic center increases there will be a reduction in signal strength observed. This could be shown to be the expected result of observation from within a disk-like structure at any location other than the center. Try to estimate the inclination of the galactic equator to the ecliptic.

Demonstrating Radio Detection of Thermal Radiation

This simple demonstration is most easily accomplished with a sensitive radio telescope (though not necessarily one of large aperture) which operates in the Vhf through microwave region. The idea is to show that blackbody radiation appears as electromagnetic energy in the radio spectrum. The concept of blackbody radiators is addressed elsewhere, but is essential to this demonstration. The radio telescope is pointed to a variety of objects including the cold sky, the ground, a human or part of one, a low mass high volume object such as a block or sheet of styrofoam, and if sensitivity is adequate the moon. Each object is targeted and the output of the telescope noted. The relationship is established between temperature and output.

By application of the appropriate formulas an estimate can be achieved for the temperature of the "quiet" sun. By quiet we mean relatively inactive sun, as it often appears in years of sunspot minimum. It is however necessary to know the characteristics of the antenna pattern and receiver quite well in order to do this. It is also important to have a knowledge of the background sky temperature. The same can be done for the moon.

Drift Scans of Point Sources

This is the activity most often envisioned in the amateur domain. The antenna is aimed along the "meridian", that is, along an imaginary line running between the celestial pole and azimuth 180 degrees (due south for northern hemisphere observers). The elevation of the antenna is adjusted to correspond to the declination of celestial object. The actual elevation will depend upon the latitude of the observer by the simple formula:

              elevation = 90 - Lat + Declination

The time that the object will pass through the beam of the antenna is determined by the right ascension (RA) of the object and the local sidereal time (LST). The object will "transit",(cross the meridian), when the LST is equal to the object's RA.

A sidereal day is the time which elapses between transits of a remote (outside the solar system) celestial object. The LST is calculated through knowledge of the observer's longitude, local time, and the relationship of the local time to standard Greenwich Mean Sidereal Time, (GMST). To fully understand these relationships it is necessary to grasp some mental image of the celestial sphere and its relationship to geometry of the Earth in orbit around the Sun. Many basic astronomy texts do a good job of explaining these terms. The time that it takes for the object to pass through the beam of the antenna depends on the antenna pattern or beam width and the declination of the source. Intuitively, as antenna is aimed closer to the celestial pole it can be seen that the angular travel of the object encompasses a smaller arc of sky. At the pole itself the arc is infinitely small and the object would not appear to move over time. As we move the antenna away from the pole the apparent motion becomes greater and thus the travel time through the antenna beam appears shorter. This relationship is defined by:

                t = 4 / Cos(declination) * HPBW

Where t is the time of the transit in minutes and the HPBW is the half power beam width of the antenna. Using the same formula is becomes possible to determine the HPBW of the antenna when the declination of the source is known and the time is measured.