RHESSI Imaging  
 

        This guide will take you through the steps toward creating a RHESSI image by using the Graphical User Interface (GUI). It assumes that your computer has already been set up to run IDL with the solar software tree (SSW)  and with access to the RHESSI flight data (See Software Installation Instructions and Accessing RHESSI Data). It is a demonstration of how to obtain a quiklook light curve plot and an image for the energy interval of 12-25 keV for a solar flare observed by RHESSI. To learn about RHESSI imaging using the IDL command line, go to Chris John-Krull's command line interface. This guide includes imaging but it does not include imaging spectroscopy.  Go to Spectroscopy-First Steps for a demonstration of how to obtain spectra. For more details on RHESSI  software go to the RHESSI data and software center.

   Selecting a flare with the Graphical User Interface(GUI)

    Start up an IDL session and type hessi on the IDL command line.

•    Click the “File” pull down button in the Hessi GUI and choose “Select Observation Time Interval”. This will open a new widget.

•      Specify start and end times. Click  “Start” and enter this date and time chosen for the demonstration: “2002, February, 20,11,0,0”. Click “Ready”. (You can also do this by typing in the times next to start and end). Another way of  doing this is to enter the flare number next to “Flare Selection". The flare number for this flare is 2022003. Next to “Duration” type in “1200”. This will change the end time to 1200 seconds after the start time.

•      Click on “Plot Observing Summary Data”. This will give you a quicklook light curve plot of  count rate vs. time in the main window. This plot allows you to see what energy intervals you want to use when creating your image. For example, in this light curve you can tell that the best images will come from using the energy intervals of 6-12, 12-25, and 25-50 keV but not 7000-2000 keV. In the next section you will create an image from 12-25 keV. Go to Kim Tolbert's Artifacts in RHESSI Light Curves webpage to learn how to interpret a light curve.

Note:   In the plot you just made, the time bins were preset to 4 seconds and 9 energy intervals were already selected for you. You can create a light curve in which you can specify your own settings by going to the top of the main GUI, clicking the "File" pull down button, and choosing "Retrieve/Process Data", "Lightcurve". For now, the observation summary plot will be sufficient but for instructions on how to use the Light Curve GUI read Creating a Light Curve.

   Making an Image

      You are now ready to create an image. At the top of the main GUI window click the “File” pull down button, and choose   “Retrieve/Process Data”, and “Image”.     
 

• In most cases the default selection for the location of the flare is correct but in sometimes you may need to find it. The default location is taken from the online RHESSI Flare Catalog. See Separate Instructions for finding the flare location. For now we will use the results of those instructions. Click “Change” next to "Offset of Map from Sun Center". Uncheck "Use XY offset from Flare Catalog" and type in “X:910” and “Y:270”.

• Notice that next to "1 Image Time Interval" the default time interval is set for 4 seconds at the peak of the flare. (You can change this by   clicking "Change" and typing in different times next to start and end.) For now we will keep this interval. Also, notice that 12-25 keV is the energy band selected, as mentioned previously.

• Check to make sure that only detectors 3-8 are selected. For now, do not use collimators 1, 2, and 9. Click “Change” to select these collimators if they are not already selected and make sure that only the box next to “Front” is selected. To see frequently asked questions about which detectors to choose look at "How do I determine which subcollimators to use when making a map?" in RHESSI FAQ.

• Next to “Image Algorithm”, the “Back Projection” reconstruction process should already be selected. Next to "Make Image(s)  and send to" check the "GUI" box. Click “Make Image(s) and send to” and after a minute or two you will see the image.

• The rings around the bright source are sidelobes.  You can remove them by changing the "Image Algorithm" to "Clean"  in the "Imaging" widget. Click "Set Parameters" next to Clean and a new widget will pop up. Change the "Max # iterations" to 100 and "Accept". Click on  "Make Images and send to" again and you will get a CLEAN image. The line down the image is the edge of the Sun.

• You can make this image a different color by clicking on the "Plot Control" pull down button and selecting "Image Colors".
   

• Select "Hessi color" and click "Accept".

This completes the demonstration. You should go back and try to change the image by choosing different detectors, pixel size, time intervals, and reconstruction techniques (MEM, Pixon, or forward fitting). You can also make images for different energy ranges and multiple time intervals. Note that plots made in any GUI session are saved until you end that session. You can find them under window control in the main window. You can also save plots in PS, PNG, TIFF, or JPEG format by clicking "Create Plot File" in the "File" pull  down  menu.

last updated:  23-Aug-2004