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Last modified: 12 June 2012

URL: http://cxc.harvard.edu/iris/v1.1/threads/plot/thread.html

Visualizing SED Data in Iris

Iris Threads



The Iris GUI is an adaptation of the Specview (STScI) spectral visualization and analysis GUI, and as a result is equipped with several of the data display preferences and editing capabilities offered by that application. This thread presents the various options available for interacting with and customizing the SED data display in the Iris Visualizer.

Note: As most Specview features remain unchanged between the standard and the Sherpa-enabled Iris version, the documentation provided in the Specview help documentation serves as an exhaustive reference for most of the data visualization features in Iris.

Last Update: 12 Jun 2012 - updated for Iris 1.1



When SED data is read into Iris, it displays in the Iris Visualizer, where you can interact with the data plot in various ways. The options available for customizing the data display are described in this thread.

[Iris screenshot image]

When multiple segments and/or photometric points have been read into the Iris session and coplotted in the main display, it is important to note that the spectral data are not combined, coadded, or spliced in memory in any way; the raw data from the multiple input spectrograms/points is completely preserved in the resulting combination. This means that two separate SED data segments loaded into Iris for a particular source - e.g., observed with two differeny observatories and spanning separate (or overlapping) spectral ranges - may be offset from one another, as Iris does not scale, trim, and join the separate segments to form a seamless SED for the source. This is illustrated in the image below.

Iris GUI snapshot[]

Setting Display Preferences

Adjusting the coordinates view port

The set of widgets in the upper-left corner of the data display are available for adjusting the view and orientation of the SED segment(s) within the main display.

[Iris screenshot image] Auto/fixed coordinates - automatically scale the coordinates view port to fully encompass the data each time the plot is refreshed; or, fix the coordinates view port so that all subsequent plot refreshes that result from a data change will take place on a fixed coordinates view port.
[Iris screenshot image] Reset - set the coordinates view port to a full view of the data.
[Iris      screenshot image] Reset to central data - set the coordinates view port to a central view of the data, with edges discarded. The central third of the wavelength span of the data is used to normalize the view port.
[Iris screenshot image] Back - return to the previously used coordinates view port. Successive application of this function causes a walk back throughout the coordinates view port history.
[Iris    screenshot image] Zoom out/in - zoom out/in by 20%
[Iris screenshot image] Move left/right - move the coordinates view port to the left/right
[Iris screenshot image] Expand - expand the coordinates view port in the X direction only.

Refreshing the Data Plot

[Iris screenshot image]

The "Redraw" button is available within the Iris main display for refreshing the data plot, e.g., to clear any visual garbling or crosshair cursor leftovers which arise under certain combinations of computer platform and CPU speed.


[Iris screenshot image]

The preferred units for the data display, e.g., "ergs/cm2/s/angstrom" versus "angstrom", may be set using either the "Units" or "Flux density" buttons in the upper-right corner of the main display.

The "Flux density" selection box allows you to choose the type of spectral quantity to be displayed on the Y axis, either "Flux density" or "Flux", and also enables the selection of units for the spectral axis quantity. Once a selection is made, a dialog box populated with all available physical units appropriate for that quantity, will pop up (this pop-up box matches the "Units" selections). Selecting the desired units and clicking on the "Apply" button adjusts the data display to the desired preference.

[Iris screenshot image]

Axis Scale

[Iris screenshot image]

The scale of the data plot axes in the Iris main display can be changed using the "Plot Type" button, or by clicking the cursor near any one of the four corners of the display; this will open a small window in which the X and Y axes scale may be set to linear or logarithmic (regular or extended).

You can also set a preference to specify that data plots should automatically display with logarithmic scaling by checking the "XAutoLog" and "YAutoLog" boxes in the Preferences->Edit Preferences window (or by using the "Auto log scale" option in the Iris Preferences pull-down menu).


[Iris screenshot image]

Selecting the "Grid" option in the main display overlays a coordinate grid onto the main plot.

[Iris screenshot image]

Displaying Metadata and Data Values

Clicking the "Metadata" button in the upper-right corner of the Iris display opens a window containing two separate tabs of information, one displaying the metadata associated with each SED data point in the plot, and the second the independent and dependent data arrays defining the plot.

Metadata tab

[Iris screenshot]

When available, the Metadata tab includes many useful pieces of information about the data points currently displayed. When a SED is imported from NED, as in the example considered in this thread, the metadata includes such things as the bibliographic reference code for each data point, the spectral range covered by instrument used to obtain data point, data point uncertainty and flux values as they are published, data point significance values, among other properties. The full list of metadata information available for each data point in this example includes the following:

  • ID - data point identification number
  • Data Reference Code - bibliographic reference for data point
  • Data Significance - data point uncertainty information, if available
  • Data Spectral Pass Band - spectral range covered by instrument used to obtain data point
  • Data Spectral Published Value - spectral coordinate of data point as it is published
  • Data Flux Published Unit - flux unit of data point as it is published
  • Data Target Position - Ra and DEC coordinates of source target associated with the data point
  • Data Spatial Mode - spatial mode in which flux value of data point was calculated (e.g., "flux integrated from map" or "total flux")
  • Data Comments - miscellaneous helpful comments, e.g., "From new raw data" or "Extinction-corrected for Milky Way"
  • Data Flux Unit - flux unit of data point
  • Data Qualifiers - miscellaneous data qualifier information, e.g., "S/N=99"
  • Data Flux Published Value - flux value of data point as it is published
  • Data Frequency Mode - frequency mode in which flux value of data point was calculated (e.g., "Broad-band measurement")
  • Data Flux Published Statistical Error - error on the flux value as it is published

The metadata table may be sorted by clicking the header of the column by which you would like to sort - once for ascending order, twice for descending order, and three times to restore the default sorting - and rearranged by clicking and dragging columns left or right. A nested sort may be achieved by first selecting the column which will set the master sort, clicking and holding the Control key, and then selecting the column by which to sort the groups of the master sort.

Data tab

[Iris screenshot]

The X and Y coordinate values of each SED data point in the Iris display are contained in the "Spectral Axis" and "Flux Axis" columns of the Data tab in the Metadata window, and reflect the values as they were imported into Iris from NED or uploaded from a file on your hard disk - i.e., not what is currently plotted in the Iris display (in the event that you changed the units of the data plot within the display). For example, if the data were uploaded in Jansky flux units versus frequency in Hertz, but then you change the display units to ergs/s/cm2/Angstrom versus Angstrom, the data point values returned will be in Jansky flux units and Hertz.

Masking SED Data Points

In order to mask or 'gray out' selected data points in the Iris display, simply select the rows corresponding to these points in either of the Metadata or Data tabs of the Metadata window, and then click "View un-selected" at the bottom of that window. In the display, the points selected in the Metadata window will appear fainter than the unselected points.

[Iris screenshot] [Iris screenshot]

The "Select from plot" option in the Metadata window may be used to select all the data points in the plot, highlighting all rows in the metadata table.

It is also possible to view a selection of data points in the display without technically masking or hiding the remaining points, simply by clicking and dragging with a mouse cursor over the plot to draw a box around the desired points; this action effectively 'zooms in' on this subset of data.

[Iris screenshot] [Iris screenshot]

Extracting a Filtered SED

The "Extract" function in the Metadata window allows you to go one step further than simply masking unwanted data points; it allows you to extract a whole new SED consisting of only the selected data points.

[Iris screenshot]

Making the desired row selections in the Metadata window and then clicking "Extract" will open a new SED in the SED Builder window named "FilterSED" - an ID which you can change - which will display in the Iris Visualizer.

[Iris screenshot]


08 Aug 2011 updated for Iris Beta 2.5
26 Sep 2011 updated for Iris 1.0
12 Jun 2012 updated for Iris 1.1

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Last modified: 12 June 2012
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