Small Radio Telescope Lab #1
AY 361 -- March 26, 2003
During this first lab, you will familiarize yourself with the Small Radio Telescope and its control program. Some of the data you will take will be analyzed for a short Lab Writeup due next week. You should frequently consult the SRT manual handed out in class.
Each group will have approximately 60 minutes to complete the parts I-IV of the lab, which must be done controlling the SRT telescope. Please take turns controlling the telescope. The rest of the lab time can be used analyzing the data with IDL and beginning your lab writeup. Each student must do their own analysis and turn in a separate writeup.
First let's calibrate the receiver. Setup the frequency using the 'freq' button (in text area type 1415 5 0) to take 5 samples at 1415 MHz, w/o scanning the LO through multiple frequencies.
You should use the 'azel' button to point the SRT antenna at the sky, at least 20 degrees from the Sun (but at the same elevation if possible). This slew will take a few minutes.
Execute the automatic calibration sequence by hitting 'calib' button. This should result in a system temperature around 240K. Information needed for lab writeup: Temperature Load assumed to be 300K, T_spillover assumed 20K.
II. Find Sun
Click on 'Sun' and hit 'track' button. This should start tracking on the sun. You should see a lot more power on the digital power recorder. Due to errors in the pointing model, the telescope may not be pointed directly at the sun. Using the 'offset' button, try to find the (az,el) which maximizes the observed antenna temperature (offset within 1 degree in alt/az is fine). IMPORTANT: the OFFSET position is an offset from the nominal direction the computer thinks the sun is, and is NOT an offset from the current position. Note the offset and temperature observed for your lab writeup. At the end of these instructions is a grid you can use to keep track of your readings (you should not have to fill very much of this large grid before finding the peak).
III. Solar Scans
You should start recording your data so that you can later use IDL to plot the scans. Use the 'record' button and give your output file a unique name. You should check that data is being recorded by checking the bottom-right part of the screen (in red text).
First do an azimuth scan. Offset the telescope -10 degs azimuth from location of peak antenna temperature. Once you see data has been acquired (in the digital chart recorder), offset to -8 degs azimuth. Continue in steps of 2 degrees until the telescope is offset +10 degrees in azimuth from the peak location.
Stop recording this data (hit 'record' button a second time) and check the file to make sure it did something. Look at the data format and, using the SRT manual, try to understand what each column of numbers means.
Return the offsets to the peak location and repeat for the elevation direction (create a new file for the elevation scan).
IV. Transfer Files
Transfer these two files using floppy disk to the astroclass account on the windows machines. Place in the directory My Documents\AY361\SRT files (you probably should make these Read Only to protect them from accidentally erasure).
V. IDL Analysis
Please spend the rest of the time left in class starting your IDL analysis and beginning your lab writeup.
Questions to answer in the Lab Writeup.
1. Explain the automatic calibration procedure used by the SRT. What assumptions are made? Are these good assumptions? Suggest a better calibration method.
2. What offset location did you find the maximum solar flux? What was the antenna temperature you found and how does this relate to the true brightness temperature of the sun?
3. Use IDL to read in data from your files. I suggest you create a personal copy of the group's data file and edit the text (using the IDL editor) to make the file easier to read. For instance, you could eliminate multiple data lines taken at the same offset position (e.g., see azscan_monnier.rad and azscan_monnier_edited.rad). Include printout of the ascii data files you used at the end of your lab writeup.
4. Plot the azimuth and elevation scans with properly labeled axes and curves. Fit Gaussians to the data and report the Full Width at Half-Maximum (FWHM) measured in each direction.
5. Explain the measured sizes based on the known diameter of Sun, the expected beamwidth of the SRT antenna at the observing frequency, and the elevation of the Sun during your observations.
6. Considering the relative sizes the antenna's main lobe and the sun, what would you estimate the true temperature of the Sun is based on the measured antenna temperature. Explain.
Extra Credit: Include error bars for each of your measurements. The error can be estimated based on the observed scatter of multiple measurements (using STDEV function).