RHESSI Imaging - First Steps

Last updated:  01-Jul-2005

This is a beginner's guide to RHESSI imaging using the RHESSI Graphical User Interface (GUI). It assumes that your computer has already been set up to run IDL with the solar software tree (SSW) and access to the RHESSI data (see Software Installation Instructions and Accessing RHESSI Data).  To learn about RHESSI imaging using the IDL command line, see An Overview of the Command Line Interface.   Imaging spectroscopy is not covered in this demonstration; further analysis is required to obtain spectra for different areas within an image. See Spectroscopy-First Steps for a demonstration of how to obtain spectra. For more details on RHESSI software refer to the RHESSI data and software center.

For this demonstration, we have chosen a flare that occurred on February 20 at 11:00 UT.  We demonstrate how to obtain a quicklook light curve plot for this flare, and an image for the energy interval of 12-25 keV for 4 seconds at the peak of the flare.

To get started, enter an SSW IDL session and type hessi on the IDL command line.

Selecting a Flare Time Interval

First we'll take a look at the time profile of the flare.  This is not required for making an image, but is recommended so that you can check the status of various indicators (e.g. day/night, attenuator state, SAA) and check the energy ranges in which the flare can be seen.  We'll use the observing summary data (pre-binned to 4-second intervals, 9 energy bands) since it's quick.  To see a time profile at different time/energy resolution we could use the lightcurve or spectrum interface (Creating a Light Curve or Spectroscopy-First Steps), but that's usually not necessary at this point.



   Making an Image

      Now we'll create an image using the 12-25 keV energy band for 4 seconds at the peak of this flare. Click the File pull-down button on the main GUI window, and click  Retrieve/Process Data, and Image.   The Imaging widget will appear:















In this image you can begin to see the double-source nature of this flare that has made it so interesting to analyze. You can still see some arcs of circles centered on the location of the spin axis that indicate discrepancies in the flat-fielding process and in the point-spread functions that are used in the CLEAN process.

This completes the demonstration of the basic technique for obtaining RHESSI images. Now that you have been through the basic steps, you can try to refine the image by choosing different detectors, changing the pixel size, using different time intervals, and using one of the other image reconstruction techniques - a Maximum Entropy Method (MEM Sato or MEM VIS), Pixon, or Forward Fitting.  Be aware that only Clean and Pixon give reliable results currently, and Pixon can take hours to complete.  You can also make images for multiple energy ranges and multiple time intervals and view them as a movie, or use them for further analysis via imaging spectroscopy.  For more complete information about the options in the Imaging widget see HESSI Image Widget.

Note that all of the plots made in any GUI session are available for the duration of that session. You can find them under "Window_Control" in the main GUI window. You can also save plots in PS, PNG, TIFF, or JPEG format by clicking "Create Plot File" under the "File" pull-down  menu.




Last updated:  01-Jul-2005