M1 is a supernova remnant. Here you will work with m1_600s_red_01d.fits
and m1_600s_01_d.fits. The red image records the
light from the hydrogen alpha line at 656 nm. The other image
is unfiltered, but will primarily detect the blue-green light emitted
by synchrotron radiation from electrons spiraling in the magnetic
field surrounding the pulsar at the center of the nebula. Use the unfiltered
image for both the blue and green channels, and the red image for the
red channel. Combine the images (beginning with ds9 -rgb as you
did for M42. When you have the hydrogen filaments clearly distinguished
in red light, save a TIF version of the color image.
The data for M1 illustrate that with multicolor imaging what you see depends critically on the balance and selection of image parameters for each color. You can make an image for which the red filaments are distinctly visible and glow in red, while the remainder of the nebula appears white, by selecting a narrow range of red values with the scale menu.