Calibrating the Telescope

Intro level: bandpass only

Usually I'm just having them measure galactic rotation or the antenna beam width on the Sun, neither of which requires a precise Tsys calibration. But galactic rotation does require a good bandpass calibration. For this I just have them use the automated routine, which does bandpass beautifully (but not Tsys, as far as I can tell). The procedure is:

1. Point the telescope at a patch of blank sky. Usually about 45 degrees altitude in the north works pretty well.

2. Hit the "cal" button on the software interface. It will say "Place absorber enter:"

3. Place the absorber in front of the receiver. I have an "absorber on a stick" that a student actually just goes out and holds in front of the receiver, making sure that as much of the antenna coil is covered as possible.

4. When the absorber is in place, have another student hit "enter." This will start the calibration. As soon as it starts recording data, you're good to go. If you're still pointed at blank sky (sometimes you have to move the antenna down to the horizon so that the student with the absorber can reach the receiver), the bandpass should now look flat, modulo some RFI.

Advanced level: bandpass and Tsys

This is what I have students do if we actually care about measuring antenna temperatures -- for example, if we're trying to measure the brightness temperature of the Sun or Moon, or doing an aperture efficiency calculation on Cas A.

1. First, do the bandpass calibration using the automated routine as described above.

2. Then, move the antenna to a blank patch of sky, preferably at the altitude of the object you're planning to observe (for stability of measurements). Press "record." In the SRTN readout in the terminal, every time a spectrum is recorded, a power measurement is recorded with it, printed to the screen in the terminal. Wait until about 4 or 5 of these measurements are made, then average them together. This will be our "cold load" measurement P_cold, and we generally assume that blank sky is 3K - this is our T_cold. (It is, of course, possible to do a sky dip and measure the atmospheric opacity to calculate the sky contribution, but I haven't found it necessary to bother.)

3. Then, place the absorber in front of the receiver and record the total power measurement the same way you recorded P_cold. This will be our "hot load" measurement P_hot, and we generally either have a thermometer recording ambient temperature, or look up the current ambient temperature on the NOAA website. A thermometer is preferable, and one can be found in the physics lab. Attach it to a portable vernier interface and bring it up to the roof with you.

4. Once you have hot and cold load measurements, you can do a Y-factor calculation to get Tsys. Specifically, if Y = P_hot / P_cold, then Tsys = (T_h - Y*T_c) / (Y-1), where T_h is the hot load temperature (the ambient temperature on the thermometer), and T_c is the cold load temperature (3K). From this, you can also derive a gain factor to apply to any future power measurement to turn the power from the telescope into an antenna temperature. For example, P_hot = G * (Tsys + T_h) allows you to calculate G, and then you can generalize so that P = G * (Tsys + Tant) to turn any power measurement P into an antenna temperature Tant.

^^The above was taken from an email sent from Meredith the SRT Hero at Wesleyan.

If you are doing the advanced calibration, once you get your new TSYS value, you'll have to exit out of SRTN, go into the srt.cat file and input a TSYS line like TSYS 178, and comment out the other Tsys's with *.