Observing Cassiopeia A

So, you feel like you know your way around the SRT and are ready to look at something other than the Sun. Great! May I suggest the lovely supernova remnant Cas A? Here's how you would go about spying on our cosmic neighbor:

Calibrate the telescope.

• First thing you will have to do before any observing will be to calibrate the telescope. For observing Cas A, you're going to need to perform the "advanced" calibration procedure.
• Make sure that your center frequency is set to something that is OFF THE H1 LINE, 1415.00 MHz works well. Your center frequency must be off the H1 line because Cas A lies pretty much right in the galactic plane, so any H1 emissions would muddy up the Cas A signal.
• Refer to the calibration page for details on how to carry out the advanced calibration.
• Once you get your new Tsys and input into the srt.cat file, you're ready to observe!

Observing Cas A

• Once you've closed SRTN and input your new Tsys, fire SRTN back up.
• Change your center frequency to 1415.00 MHz or whatever other frequency you are trying out.
• Do a regular calibration procedure, aka just the bandpass calibration that comes boxed in SRTN.
• Once the telescope is calibrated, slew over to Cas A.
• Hit record.
• Hit the "beamsw" button on the top bar of buttons in SRTN. This will start the beamswitching procedure, which takes the telescope on and off Cas A by a beamwidth on either side. This technique is used to tease out faint signals.
• Wait till about 40-45 beamswitches pass. There is a counter in the average spectrum (bottom spectrum) window for you to keep track of.
• Hit "beamsw" again to turn the beamswitching off.
• Hit record again to stop recording.
• Stow the telescope and close SRTN (unless of course you aren't done looking at stuff!).

Parsing and Plotting the Spectra.

• Now that you have your output file, you're going to need to parse out the spectra so you can plot them.
• To parse out your data file, move your file to the home directory of the radiotelescope computer.
• In the home directory there also lives the parsing programs. The spectrum parser is called spectrum_parse.py. Check out the parsing program usage page for details on how to run the program.
• Once you have your spectra parsed out and written to a new file, you can start to plot them all.
• First, open up the original output file you made. This is sort of tedious but you're going to need to look at the spectra (large blocks of numbers) to see which ones if any pop out at you. Generally if the numbers that make up a certain spectra are larger than the numbers of the other spectra, those are the ones you're looking for. Alternatively, look for the spectra that were taken at the Az/El of the source. You'll be able to tell because the spectra will always come back to a certain central Az/El and then go off to one side azimuthally. Count how many spectra into the file your spectrum of choice is - this number plus 1 is going to be the column number of your spectrum in the parsed file. Hold on to this number -- you will need it when plotting.
• Now that you've picked out the spectra you want to plot (it is a good idea to plot off-source spectra for comparison), you can actually plot the spectra.
• Fire up gnuplot and plot away. Refer to the gnuplot usage page for details.

Understanding, Interpreting, and changing the units of your plots into something useful.

• So the y-axis in your plot will have units of Kelvins. The numbers in the spectrum correspond to the power level measured at a given frequency, and these numbers are given in Kelvins. We want to convert Kelvins to Janskys, a unit widely used in radio astronomy to quantify spectral flux density. Most if not all established Cas A spectra are given in Jansky's v. Frequency, so we shall follow suit. This page has lots of great basic radio astronomy info and has a Kelvin to Jansky conversion formula, reproduced here:
• Ta is Antenna temperature, or the values we have in Kelvin right now. S(nu) is spectra flux at a certain frequency, which is what we're looking for, so get S(nu) by itself. A dish is area of the dish, which is a 2.4 m diameter dish. You can figure out the area from there. k is Boltzmann's constant, which can be easily found on the internet or any physics book. Plug your Tant value at whatever frequency you're interested in into the formula and calculate away to get the spectral flux density at that frequency. The value you get at this point will be in J/s/m^2/Hz, and 1 Jansky = 1x10^26, so you'll need to multiply the value you just got by 1x10^26, and then you'll have your spectral flux density in Janskys. Now you can compare to established continuous spectra of Cas A to see if you really saw it!