Many moons ago, when I was an astronomy undergraduate, I had a course entitled, “Observational Techniques.” One of the lab practicums that were given to us was to use the 26-meter radio dish atop Peach Mountain to make a map of the Sun. The Sun is an extended object, that is, it is not a point source. Therefore, one must make a series of “slices” across its face in order to cover the entire object. That may sound simple enough, but bear in mind that the Sun appears to move across the sky. If this motion is not taken into account, you’re going to end up with a warped map.
First, one has to determine the “daily motion” of the Sun, i. e., the rotation of the Earth. This amounts to 360° per 24 hours or 1 arcminute every 4 seconds.
Next, to take into account is the “yearly motion” of the Sun, otherwise known as the revolution of the Earth around the Sun. This is equal to 360° every 365.25 days or about 1 arcsecond every 24 seconds. For this motion, however, one also needs to take into account the fact that the Earth is tilted 23.5° with respect to the ecliptic (the plane of the Earth’s orbit). Therefore, the declination motion must be multiplied by the sine of 23.5° or 0.399, and the right ascension must be multiplied by the cosine of 23.5° or 0.917.
The resolution of the radio telescope depends on both what wavelength one is looking at and what the size of the telescope is. We were using an 8-gigahertz feedhorn, which corresponds to a wavelength of 3.75 centimeters. The ratio of this wavelength to the antenna width yields a resolution angle of about 5 arcminutes. (At the Sun’s distance, this represents a width of just over 200,000 km.)
The actual scanning process entailed four students - one to move the antenna, one take readings off the chart recorder, one to record the right ascension and declination of each measurement and one to note the times of the readings (yours truly). For each sweep, measurements were taken at 1000 K intervals.
There is a second feedhorn on the telescope. Its purpose is to monitor the sky background for comparison. The background is then subtracted from the original map.
After all the sweeps were complete, we made the radio map by taking the raw positions, subtracting the appropriate locations differences from the R. A. and Dec. coordinates due to the motion of the Sun, and then plotting the intervals and connected like temperatures with contour lines. Note: when you do this yourself, remember that declination minutes and seconds are not the same sizes as right ascension minutes and seconds. We realized this just after making our first football-shaped Sun.
Anyhow, you can see results here. As you can see, we were fortunate to have a very prominent sunspot available for our project.
My thanks go out to George Latimer for his help in the writing of this article.
Radio map (hand drawn) of the sun. All points and lines were plotted from data collected by Doug Warshow and three other students. The contour spacings are ~1000K. R. A. and Dec. are plotted.