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Histograms

Lastly, we will demonstrate one other powerful feature of fv which allows you to construct 1-D or 2-D histograms from columns of a table. First, close the POW window if it is already open, go back to the table spreadsheet window for the rosat.evt table, and click on the Histogram menu. From the histogram control panel (Figure 19) you can construct a 1-D histogram showing the energy spectrum of all the events in this table by entering `PHA' for the X column then click on the `Make' button. This will create 2 new windows: one is a Summary window for the new temporary file containing the 1-D image, and the other is a POW window showing a plot of the PHA spectrum (Figure 20).


 
Figure 19: Column histogram control panel
\begin{figure}
\begin{center}
\leavevmode
\epsffile{histo.ps}
\par\end{center}\end{figure}


 
Figure 20: Histogram showing the PHA spectrum
\begin{figure}
\begin{center}
\leavevmode
\epsffile{powhisto.ps}
\par\end{center}\end{figure}

Now go back to the Histogram control panel, enter `X' for the X column and `Y' for the Y column and again click on the Make button. This will generate another Summary window for the new temporary file. Click on the `Image' button to display the X-ray image of the sky. The image is unfortunately rather underexposed in this case because this sample FITS file has been reduced in size to save space, and only contains a total of 2900 events, compared to many 10s of thousands of events in a more typical ROSAT observation.


next up previous fv Home
Next: Summary Up: No Title Previous: Table Analysis and Plotting

Project Scientist: William Pence
Project Engineer : Pan Chai
July 2008