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.