Small Radio Telescope Lab #2
AY 361 -- April 9, 2003
During this lab, you will familiarize yourself with the spectroscopic capabilities of the Small Radio Telescope and apply them to learn something about the HI distribution in our galaxy. You might need to 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 due on April 16.
You will measure the total HI emission (integrated over all velocity components) as a function of galactic latitude at one particular galactic longitude. From this data, you might discover that the HI in the Milky Way is not isotropically distributed.
I. Choose Galactic Longitude and Calibrate Receiver
As usual, you must first 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. This part of the spectrum lacks any strong line emission and thus is suitable for calibration.
Next, you will use the 'azel' button to point the SRT antenna at the sky. First decide which galactic longitude you wish to scan in latitude. During your lab, the galactic plane lies almost entirely along the elevation axis, hence the azimuth axis is almost the same as the galactic latitude direction. Unfortunately, in order to point the SRT from the java GUI, one can not specify the galactic coordinates but must estimate the appropriate azimuth/elevation from the SRT display map (however, your pointing direction is reported in galactic coordinates in the upper-right of the display). Once you are pointed at the desired part of the galactic plane (remember that this might take many minutes), execute the automatic calibration sequence by hitting 'calib' button. This should result in a system temperature around 240K.
II. Setup Receiver
Unlike the Solar Scan experiment, we want to use the spectroscopic capabilities of the SRT. Setup the frequency scan to be centered at the rest frequency of the 21cm line (1420.4 MHz) and with enough channels to cover all galactic plane emission. This can be done using the 'freq' button (parameters 1420.40 51 0.040). This will now readout 51 spectroscopic channels, each separated by 0.04 MHz, and centered at 1420.4 MHz.
If you are pointed in the galactic plane you should start seeing an emission line spectrum on your screen. The right panel is the data from the most recent scan and the left panel is the accumulated (time-averaged) spectrum. Note that to clear the accumulated spectra you must click the 'clear' button. If you do not understand this section, please see the instructor for help -- otherwise all the "data" you take might be garbage.
III. Collect Data
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). I suggest using a separate file for each galactic latitude you are measuring, in order to make data analysis easier later (if each file only contains a single pointing, you can use a prewritten program to read-in data -- see Tips for IDL Analysis). An example filename would be "group1_lat30.rad".
After starting data taking, hit 'clear' so you can start accumulating a new spectrum. While you wait for data collection, fill in the data log (at the end of these instructions) -- note the time, telescope azimuth and elevation, galactic longitude and latitude. Note any unusual problems in the log as well (such as interference SPIKES which seem to happen around 1420.04 MHz). When you are done taking data (after a few minutes), hit 'record' button to stop data-saving. Also, note in the log what "Max - Min" Temperature was: this is displayed in the time-averaged spectra display. This is a crude measure of the HI column density/optical depth.
Now point the telescope at a different galactic latitude but nearly the same longitude. Make sure your latitude measurements are separated by at least 10 degrees so that you collect a wide range of latitudes (for example, collect data at latitudes -30,-20,-10,0,10,20,30,40,50... until you run out time). Before starting a new data file, check that you are pointed at the desired galactic coordinate by checking display (remember that a certain change in azimuth angle does not directly translate into the same amount of angular change on the sky, depending on the elevation angle). Remember to fill out your Data Log sheet with the correct observing parameters.
IV. Transfer Data
Transfer your 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.
Your lab writeup should completely explain what you did during the lab, how you analyzed your data, and a presentation of your results. Importantly, you should attempt to explain your results in terms of the galactic distribution of atomic Hydrogen. An example of an excellent lab writeup of a similar experiment can be found at http://www.lanssiers.be/radio/report.html (Your lab writeup does not need to be this extensive!)
At the minimum, your lab writeup should contain the following (see Tips for IDL Analysis below for further help).
Tips for IDL Analysis
I have provided a function called "read_srt" which can be used to read-in your data. Here is an example of how to use it:
IDL> read_srt, freq, temperature
After typing the above, you will be asked to choose a file. The program will then read in your data and fill the variables 'freq' and 'temperature' with the median values in MHz and Kelvin respectively. You can see your data easily:
IDL> plot, freq, temperature, xtitle="Frequency (MHz)",ytitle="Antenna Temperature (K)",psym = -4
While the INT_TABULATED function in IDL can be used to integrate the total brightness, you must first subtract an estimate of the background level in each spectrum. Estimate the slope (m) and y-intercept (b) of this background line by inspecting the previous plot of temperature vs. frequency. Next, interpolate the background into the same frequency grid as the data:
IDL> background= b+freq*m
and then subtract it:
IDL> corrected_temp = temperature - background
Always check result (IDL> plot,freq,corrected_temp) before integrating! If you see there are big data spikes that you think are not "real" but come from interference (as noted by in your Data Log -- often near 1420.04 MHz), then you can set these channels (by hand) to zero or to the value of neighboring "good" channels. Once you have a good background-subtracted HI spectrum, then Numerical Integration is easy in IDL:
IDL> result = int_tabulated(freq,corrected_temp)
After recording this result and the latitude of the observation, repeat the process for each datafile/latitude. When complete you should be able to then plot the integrated brightness vs. latitude.