The spectral processor can be used as a sensitive spectrum analyzer for RFI measurements when it is not in use for astronomy at the 140-ft telescope. This note describes how to start the "spectrum analyzer" control panel and an associated aips++ session to look at the spectra.
Before opening the connection to the spectral processor make sure that it is not in use and is not scheduled for use while you expect to be using it.
There are lots of combinations of workstations and remote logins that can be used on the Green Bank network to run the spectral processor. We haven't established a "best" way, yet, so the following procedure may change from time to time as the GBT computer system develops, and we find better ways to automate this start-up procedure. I'll assume that you are sitting at a PC running X windows and that you will log into the Sun workstation, rigel, running the Bourne or bash shell with the '$' prompt. If you are using one of the c shells, the bash command
export variable_name=value
translates to
setenv variable_name value
If you are already running on a UNIX workstation capable of running
aips++, you can skip the xhost and rlogin lines below. We'll create a
directory, called "Analyzer" for the purpose of having someplace to put
files created by aips++, but it could be any directory. In an X window
type
xhost +
rlogin rigel
(plus any password that it asks for)
$ mkdir Analyzer (skip if you already have this directory)
$ cd Analyzer
$ export DISPLAY=< PC's network name >:0.0
$ . /GBT/gbt.bash
$ glish -l $GBT_TELESCOPE/glish/spectrum_analyzer.g
This will ask you to verify that the spectral processor is not in use by
someone else, and then it should open a "Spectrum Analyzer" control panel on your screen. If you get a "glish:
command not found" error, you probably need to add /opt/local/bin to
your path.
$ export PATH=$PATH:/opt/local/bin
After a little while you should also see an "AIPS++ Log Messages" window
pop up on your screen, which you can minimize to get it out of the way.
After you run a spectral processor scan, or if there is an old scan in
your directory, a plot window will pop up to display
the spectrum. You will want to expand this window to make it easier to
read.
To close your session type exit at the glish prompt.
All of the setup information required to run the spectral processor as a spectrum analyzer is shown on the control panel. You can get a brief explanation of each entry field or button by clicking on its label. All of the fields have reasonable default values so, if you don't know what to set something to, it's probably OK as is. Fields that aren't "sunken" cannot be set. They're just for your information. After you have filled out the entry fields and selected the button choices, press the "Start" button to start a scan.
Normally, you will use the Total Power switch mode. This asks you to run two scans, first with the source of potential RFI turned on and then with it turned off. After the first scan you will get a plot of the spectral processor's noise spectrum plus any RFI on top of it. After the second scan you will see a plot of the difference between the RFI on and RFI off spectra normalized to the total system noise power. Use the zoom and axis buttons on the plot window to rescale or look at particular spectral features.
rfisher@nrao.edu