True Color Images in ds9

Creating Fluxed Input

If there is significant exposure variation across your data, e.g. you are using both front- and back-illuminated ACIS chips or are working with a mosaic, consider creating fluxed FITS images as input for the true-color image.

To create fluxed input images:

1. Filter the event list into three energy bands.

2. Create an exposure map for each filtered event file by running the fluximage script, as described in the Single-Chip or Multiple-Chip ACIS Exposure Map thread.

   After completing the thread, you will have three normalized images, one for each energy band.

3. Use these fluxed images as input to this thread.

While creating exposure maps adds a non-trivial amount of time to the analysis, the image results can be significantly improved.

A. From the command line

The ds9 command-line syntax can be used to create the RGB frame and load the three filess in the correct bands:

unix% ds9 -rgb -red red_198.fits \
               -green green_198.fits \
	       -blue blue_198.fits &

ds9 will open with the three files in one frame, as shown in Figure 1; the "Scale" is set to "log: minmax". The RGB window (Figure 2) should open as well. If it doesn't, open it from the "Frame → RGB..." menu.

Use the binning and zoom options in ds9 to adjust the image so that the full region of interest is visible. This is similar to applying a spatial filter and binning specification when creating an image with dmcopy.

Figure 1: RGB frame with three files loaded

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Figure 1: RGB frame with three files loaded

Figure 2: ds9 RGB window

Figure 2: ds9 RGB window

If this window doesn't open automatically, launch it from the "Frame → RGB..." menu.

More information on the RGB command-line options is available from the ds9 Manual.

B. From the ds9 GUI

To load the files from the ds9 GUI:

1. Launch ds9

2. Choose "New Frame RGB" from the "Frame" menu.

   When the new frame is created, the RGB window (Figure 2) should open as well. If it doesn't, open it from the "Frame → RGB..." menu.

3. Make sure the "Red" band is selected in the "Current" column of the RGB window, then choose "File → Open..." in the main ds9 window and select the red (soft-band) file, red_198.fits.

4. Change the current band to "Green" in the RGB window. Open the green (medium-band) file, green_198.fits, as you did for the red.

5. Change the current band to "Blue" in the RGB window. Open the blue (hard-band) file, blue_198.fits, as you did for the red and green.

The resulting ds9 display is shown in Figure 1. The "Scale" is set to "log: minmax".

Use the binning and zoom options in ds9 to adjust the image so that the full region of interest is visible. This is similar to applying a spatial filter and binning specification when creating an image with dmcopy.

C. Locking frames

Each frame of the RGB image may have different binning, scaling, smoothing, and colorbars applied to it. You can "lock" the frames together, so that all the setting is applied to all three frames at once. This is done with the Lock menu in the RGB window (Figure 3); all five options are checked in this thread.

Figure 3: Lock menu in the RGB window

Figure 3: Lock menu in the RGB window

Overlaying a Source List

To overlay a source list on a ds9 image:

• ASCII files: load the region file from the ds9 menu "Regions" → "Load Regions..." → ascii_file.reg.

• FITS files load the region file from the ds9 menu "Regions" → "Load Regions..." → fits_file.fits.

  The region definition is saved in the REGION or SRCLIST extension of the FITS file. ds9 should automatically find the correct extension.

Read the Using CIAO Region Files thread for more information on working with regions in ds9.

Adding a Coordinate Grid

To add a coordinate grid to the image, choose the "Coordinate Grid" option from the "Analysis" menu. Then choose "Coordinate Grid Parameters" from the same menu to open the preferences dialog (Figure 6).

Figure 6: Coordinate Grid parameters dialog box

Figure 6: Coordinate Grid parameters dialog box

From the preferences box, you can change the color, font, line style (solid or dash), and line thickness for all elements of the grid. The font style, size, and color can be edited, and you can add titles. It is also possible to turn off the display of individual items via the "View" menu.

In Figure 7, we have turned off the grid lines and border. The axes have been changed to "Exterior Axes" (from the "Type" menu). The ds9 "Edit → Preferences → View → Background Color" menu was used to make the background of the ds9 window black instead of the default white; this provides a better display for the axes on the image.

Figure 7: Final three-color image with coordinate grid overlaid

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Figure 7: Final three-color image with coordinate grid overlaid