GBT spectrometer overview

The program spectrometer is primarily used to display GBT spectrometer data for near-real-time diagnostic tests. The spectrometer program has the capabilities of:
  • Displaying the raw spectrometer lags
  • Fourier transforming and scaling the lags
  • Calibrating the transformed data (using Cal on and Cal Off signals)
  • Subtracting Off Source spectra from On Source spectra.
  • Fitting a linear baseline
  • Fitting a gaussian to a spectral feature
The spectrometer program runs on any Sun workstation by typing spectrometer at the unix prompt (or /opt/local/bin/spectrometer if /opt/local/bin is not in the default search path).
Spectrometer Main Window
The spectrometer program has several sub-windows, accessed from the main window.
  • Choose FITS allows the selection of raw spectrometer data for plotting. This button brings up a list of sub-directories in the data directory "/home/gbtdata". The user is given the option of selecting a sub-directory containing GBT spectrometer data. (Spectral Processor data is also supported.)
  • Show Plot brings up another window displaying the spectrometer data.
  • Help brings up this web page.
  • Quit exits the program, closing all sub-windows.

The spectrometer program can configure the GBT spectrometer hardware, but only in the mode where the hardware is connected to the "MVME 167" processor. When in the "MVME 167" configuration, the button "Show Tests" brings up a display which shows hardware internal test messages. When in the "MVME 167" configuration, the button "Show Config" brings up another display which shows a menu of hardware internal configurations. These configurations may be loaded with these displays.

Spectrometer Plot Window

The plot window contains the two dimensional data plot (top) and several fields of buttons and user input regions. The plot window is organized to display 4 spectra simultaneously. The nature of these 4 spectra are different depending on whether the data are displayed as Raw, FFT, or TCal Calibrated is selected.

The image at left shows calibrated data for dual polarization observations of Messier 33. A gaussian fits to the neutral hydrogen line are shown. The image is produced by avering two "ON" observations of M33 using the "Source Stack" and one "OFF" observation put in the "Off Stack" (see below).

The definitions of the calibration process is described below, along with a summary of the functiosn of the user input buttons.

Raw Data

The plot at left shows a selection of the raw data from the plot window, after Fourier transforming. The four dotted lines show the two polarizations observed and the two calibration states for each of the polarizations.

This display is shown when the FFT selection is chosen in the Raw/FFT/TCal field.

These are Fourier transformed from the lag data. But the individual spectra have not yet been combined to produce calibrated spectra by the equation:

(CalON + CalOFF)*TCAL/ (CalON - CalOFF)
This is done when TCal is choosen.
Processing Input Options
  • Select Full/Zoom steps of X axis range.
  • Select Log/Linear Y axis plot.
  • Select Channel/Frequency X axis plot.
  • Display Raw, FFT or TCal calibration of the data. The Raw option is used to examine the lags before, the Fourier Transform step. The spectra may be calibrated [(On+Off)*TCAL/(2*(On-Off))], optionally.
  • Display One, Ave or Fit integragions. The One integration option allows individual integrations to be viewed. The Ave option averages all spectra in a Scan before display. The Fit option performs averaging and a fit to the CalOn - CalOff spectrum before computing the Calibrated spectra.
  • Turn on the Vanvleck 3 level or 9 level corrections.
  • Select Weighting Function of lag data before Fourier Tranforming.
  • Auto Scale of Y axis on and off.
  • Display messages concerning use of the stacks.
  • Display the Average and RMS of the data in the visible region.
  • Subtract Linear Fits to the 2/10s of the ends of the visible region (the central 6/10s of the visible region is not fit).
  • Fit Gaussians to the visible region. The fits of the two polarizations are printed on standard out, with the average and difference of the two fits.
Selection of Spectra for Display

The plot field shows the contents of up to 4 plot arrays. These arrays may be selected for display using the PLOT selection fields (at the extreame left of the input area).

In addition, some combinations of the individual spectral in the group of 4 is allowed. The 1-2 button presents the difference of the first two spectra in the group of 4. This is normally CalON - CalOFF.

1+3/2 button presents the average of the two calibrated spectra. 1->2 button transfers spectrum 1 to spectrum 2. 1->3 button transfers spectrum 1 to spectrum 3.

Accumulation of Off Spectra

The Plot function allow provides limited data processing support, by implimenting two separate spectra stacks. The Off is intended for accumulating Off-source spectra for subtraction from the On-Source spectra that are accumulated in the Stack .

  • The =>O transfers all 4 spectra from the PLot arrays to the Off stack. The count of spectra on the Off stack is reset to 1.
  • The PL<= Transfers the average of the Off Stack to the PLot arrays for display.
  • The +PL Transfers the average of the Off Stack to the PLot arrays for display.
  • The PL-O Transfers the difference of the Plot Stack and Off Stack to the PLot arrays for display.
  • The PL-O/O button is used to perform the

    sourceOn - sourceOff divided by sourceOff calculation.

Accumulation of Source Spectra The Stack is intended for accumulating On-Source spectra.

  • The =>S transfers all 4 spectra from the PLot arrays to the Stack. The count of spectra on the Stack is reset to 1.
  • The PL<= Transfers the sum (not average) of the Stack to the PLot arrays for display.
  • The +PL Adds PLot arrays to the values in the Stack. The PLot arrays are overwritten with the Stack spectra, for display.
  • The -PL subtracts the PLot arrays from the Stack. The PLot arrays are overwritten with the Stack spectra, for display.
  • The S*F scales the Stack with a scale Factor, which is a user input parameter (right).
  • The PL<< Transfers the Stack spectra 1 and 3 to the PLot spectra 2 and 4. This function allows to separate scans to be compared on the same plot.
  • The PL/S Divides the PLot Spectra by the Stack and transfers these spectra to the PLot arrays for display.
User input fields A number of calibration parameters must be entered by the observer. The Print and Plot buttons write to files. The names of the files are given in the standard output window.

The spectrometer program multi-window environment was created by Jeff Hagen and modified by Glen Langston to support the GBT FITS format data files.

The 2001 September 20