Introduction

Since you can't actually see the sky that a radio telescope is pointed at, knowledge that the radio telescope correctly points to the location in the sky that it was instructed to is crucial for any observation with the telescope. Additionally, one of the first characteristics of a radio telescope that one needs to know is its system temperature, which is a measure of the amount of noise that will appear in a measurement and which the signal from the source (measured as antenna temperature) needs to stand out from to be detected. A third important parameter that will also be determined in this lab, is the beam width, which is another way of describing the resolution of the telescope. .

Follow the procedure listed below. This will both help you become familiar with the operation of our telescope as well as make the measurements needed for this lab.

For your report for this lab, imagine that you are merely writing a document for yourself and future users of the telescope to refer to. Do not write it like a scientific report, but rather like a report for the telescope manual. So, you do not need to write an abstract. The introduction should be very short, merely stating the purpose of the report. List all results in a clear, well-organized table right at the beginning, and then follow the table with a discussion of how you made the measurements. Finally, put in a discussion in which you analyze the results; e.g., how reliable do you think the measurements are?

What is "the Beam"?

Get Familiar with SRT_Plotter

Lab Procedure Instructions

Analysis of the Data

Introduction: What is The Beam?

The beam is a radio astronomer's term to describe the area on the sky in which radiation will be detected by the radio telescope. The term derives from the fact that radio telescope engineers design the telescopes as if they are transmitters. The path of light can always be traced backward as well as forward and so modeling the radio telescope as a transmitter works, and is generally easier. Thinking of the telescope as a radio transmitter, then, radio waves are emitted out along a beam. Now, in terms of the telescope as a receiver of radiation, it will respond to the radiation coming in along that same beam. So, "the beam" of a telescope is a measure of how large an area the telescope is sensitive to. In general, a smaller telescope has a wider beam.

Now, the sensitivity of a telescope is not uniform across the beam. In most cases, the telescope is most sensitive to radiation entering the telescope straight-on, and the sensitivity fades off towards larger angles. So, radiation that enters in the beam but off-center might produce a response in the receiver 80% of that when it enters in the very center. The "width" of the beam is generally considered to be the angle between the half-maximum points (or Full-Width-at-Half-Maximum, a.k.a. FWHM).

The width of the beam is also a measure of the resolution of the telescope. If the individual telescope is used to map a source -- by charting the amount of radiation detected when the telescope is pointed in various directions -- then the inferred map will show the source to have an angular size at least as the telescope's beam. Only structures on angular scales larger than the telescope's beam can be revealed.

Procedure Using the SRT:

(Must be done during daytime -- the closer to noon the better.)
1. Turn on the radio telescope computer.
2. Turn on the radio telescope control box.
3. Click on the SRT icon in the lower left:.
An MSDOS window will open--do nothing and wait a minute or so.
Then, the SRT window will open.
Wait until the screen no longer says "slewing" (a few seconds).
4. Click on "record" (on top bar) and in the "text window" at the bottom type in a data file name.
Make sure that it says "recording yourdatafile.rad" in the lower right, at the bottom of the "information side bar".
5. Set the frequency adjustments by clicking on "Freq" and in the text bar at bottom, enter:
    1420 1
(which means L.O.=1420 MHz, operating in mode 1)
6. Click on "Sun" in the display window. You should see a yellow cursor appear at the position of the Sun and the red cursor to the lower left corner should start moving (slowly) toward the Sun. When the red cursor reaches the Sun, the yellow cursor will disappear.
7. Click on "Offset" and type in "0 15" in the text box at the bottom (this will move the telescope 15 degrees North of the Sun).
8. Calibrate the signal: Click on "Cal" and wait until a value for Tsys, in the box at the right, appears.
9. Check the pointing of the telescope:
Click on "NPOINT" and wait about 10 minutes. (The telescope will now measure the antenna temperature at 25 different positions in a 5x5 array.)
When the NPOINT is done, you’ll see a false-color plot displayed at the top of the screen.
Near the bottom of the information side bar, under "Scan Results", you should now see:
the calculated "Offset" based on the NPOINT data,
the maximum Tant,
and the width of the apparent image will be listed.
Write these numbers down. The computer will automatically set the pointing to the peak position. After you move to another source, though, this offset is lost.
10. Repeat step 9 to get a second measurement of this very important determination.
11. Measure the beamwidth of the telescope more explicitly by doing the following:
Click on Offset, and enter "-20 0".
Let data be recorded for about 30 seconds, then move to an Offset of -19 0, then to -18 0.
Continue all the way to an offset of +20 0.
Then do the same range of offsets in Elevation (offsets from 0 –20 to 0 +20).
12. Click on "Record" to stop the recording of data.
13. Click on "Stow" to return the telescope to its initial, and storage, position.
14. Exit the program.
15. Insert your flash drive and copy your data file from c:\SRTvsrt to your flash drive.
16. Shutdown the computer.
17. Turn off the ground control box.

Analysis of Data:

1. On a data-analysis computer (which could be your own computer) open SRT_Plotter, click "Open File," and "browse" to find the data file.
2. Click "Plot Beam," opt to "plot vs. azimuth," and then select only the data blocks corresponding to observations of the Sun with offset in azimuth (the first set of data, if you followed the procedure above). You'll then see a plot of your data.
3. To save the plot as a JPEG file, move the mouse to the plot screen and right click.
4. Note the full-width-at-half-maximum (FWHM) of the beam pattern. This is the beam width of the SRT in degrees of azimuth.
5. Now, to convert the angle in azimuth to angles of arc, you need to multiply the azimuth angles by cos(elevation). Consider that when the telescope is pointed straight up, for example, movement in azimuth merely rotates the antenna about its central axis and so moves the telescope 0o in the sky. When it’s pointed at the horizon, then movement of 360o in azimuth does actually correspond to 360o of arc. To find the elevation setting of the telescope during these scans, use Notepad to open the raw data file (the datafile.rad file written during the observation) and read the third column of numbers for the rows used in the azimuth beam scans. Measure and write down the azimuthal beamwidth in real degrees. (Improvement for future: the ability to read elevation and azimuth angles and to convert azimuth to real angles by multiplying by cos(elevation) will be added to SRT_Plotter.)
6. Convert your primary beam width to radians and compare with that given by
FWHM = 1.15 λ/D,
where D is the diameter of the antenna and λ is the wavelength of the radiation. (λ = c/ν, where ν is the frequency as determined by the L.O. setting you typed in when you clicked on "freq" in the observations (see step 4 of the observation instructions.)
7. Click "Plot Beam" again and this time opt for "plot vs. elevation" and select the appropriate data blocks. Save this graph, measure the FWHM in this direction.
8. For the discussion of your report, consider the following issues.
a. The angular size of the Sun, as seen at radio wavelengths, is between 0.5 and 1 degree. How does this compare to the FWHM of the telescope beam? If it is comparable to or larger than this, then the measured beam is larger than the true beam, and if the Sun is much smaller, than this is a good measure of the beam.
b. How does the beamwidth compare with 1.15 λ/D if D = 2.1 meters?
c. What is the "effective diameter" of the telescope according to these data?
d. How does the effective diameter compare with 2.1 meters, (which is the physical diameter of the telescope)? Why might they be different? (Feel free to ask your instructor for any ideas.)
e. Are the beamwidths in the two directions the same, within reasonable uncertainties? f. How accurate is the pointing of the telescope, in comparison to the beam width? Were the calculated pointing offsets smaller than 1/10th of the beam width?