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M42 - Orion Nebula

The famous Orion Nebula is recorded in three different color bands that we will overlay to make a full color image. Start ds9 in rgb mode with the command line ds9 -rgb. This will bring up the usual display window, and a control panel with buttons to select the current color channel. You have to load an image into each channel. By default, ds9 will start expecting you to load the Red image channel. Select File -> Open and the image m42_100s_red_02d.fits. This is the dark-subtracted image taken through the red filter. (Note that the 02d file is used for the red image because it is a better image than 01d.) You will see a red colored image on the screen when it is loaded.

Now change to the Green channel using the buttons on the little menu. Click on the Green button, and then select File -> Open and the image m42_100s_green_01d.fits. The combined (red and green) images will have an orange cast.

Load the Blue channel by repeating the process one last time: change to Blue using the Blue button, and then select File -> Open and the image m42_100s_blue_01d.fits. At this point the image will be more balanced and look approximately white. You will probably not see much detail yet.

The Scale -> Parameters menu controls the mapping of the selected channel (r,g, or b) to the display. Adjust the low and high levels for each of these in turn (selecting r, g, or b with the Red, Green or Blue buttons in the RGB menu box). At the least, set the Low level to 0 or higher to exclude the overexposed values that are read as negative numbers by ds9. Set the High value at about 6000 to add dynamic range to the image. Similarly, the Color -> Contrast/Bias setting also applies to each channel. Adjust these to create a display image that you like. Experiment with scaling: linear, log, squared, or square root as well. Remember that to see these images with North up, you should select Zoom -> Invert Y (this applies to the entire rgb set).

There will be a slight mismatch in the centering on each image due to small differences in the optical properties of each filter and slight errors in tracking during the course of the exposures. These could be corrected by separately recentering each color image using ds9 without the rgb option, sampling the centered image to produce a new image, and saving as a fits file the recentered images. Combining the carefully centered images with ds9 -rgb should produce an improved final image without the bright stars appearing to have color at the edge of the image disk. You might try this if you have time, although it is not necessary to do it in order to see differences in color across the nebula.

Finally, for any color image you want to save, zoom out to a level that shows everything you are interested in (it helps to stretch the corners of the display box to include most but not all of the screen).

Select File -> Save image as. Here you have a choice of color image formats. JPEG is a compressed image that takes less storage space but loses quality; PNG or TIFF will save all the details of the original. For web or email applications, you might use JPEG since it makes a smaller file, but here use PNG or TIFF to save a full color, full resolution, image. Either of these formats may be loaded back into Gimp or Photoshop for adjustment and touchup later, if desired.

Use these techniques to produce a full color image that highlights and identifies where the atomic hydrogen Balmer alpha (red) line emits most strongly. Save this image in TIF format without compression. When you are finished with this exercise make a note of the directory and file name for the finished images.


next up previous
Next: M1 - Crab Nebula Up: RGB Images Previous: RGB Images
John Kielkopf
2004-11-30