RGB vs Narrow Band Imaging

Figure 1    Shows the range of visible light frequency spectrum.  The White line is the light frequencies that the Luminous filter allows through.  You will also see a Red, Green and Blue lines.  They represent that portion of visible light that the filter allows to pass through.

Normally Astronomy images are taken through Luminous, Red, Green and Blue fiters.  These filters cover the broad range of the visible light spectrum.  The process involves taking several images (of the same object) and stacking them togeter.  This is done with each of the filters.  Based on the filter, different wavelengths  of visible light are allowed to pass through while blocking the rest.  The Luminous filter allows all visible  light through but can not distinguish between the Red, Green and Blue light frequencies (thus the reason for the indivudal filters).  There are software programs that are used to combine the various L, R, G, B images to make a single colored image.  There are Astroimaging cameras that are capable of taking a color image without having to use filters.  Each setup has its own advantages and dis-advantages.

       Luminous  filter                                       Red filter                                           Green filter                                    Blue filter                      Final result after adjustments

 +  +  +  =

___________________________________________________________________________________________________________________________________________________________________

Figure 2    Shows the narrow range of light frequencies allowed through each of the filters.

Narrow Band Images are taken through special filters that only allow a narrow range of visible light frequencies to pass through.  These images are known as false color images.  Nebulas are comprised of various gasses and dust.   The filters mostly used are the Ha, SII  and OIII (Hydrogen, Sulfer, Oxygen) .  Besides being used to determine the types of gasses in the nebula, the images produce finer details.  On earth, these narrow band filters eliminate or reduce the effects of light polution on the final image.  The Hubble telescope uses narrow band H, O, S filters also.

When combining these three images, each one is assigned to a basic Red, Green or Blue channel but because these filters limit the color allowed through, a unique final color image is produced..  The images below show each of the before and after combining process.

Ha  assigned to Green channel        OIII assigned to Blue channel         SII  assigned to Red channel         Final result after adjustments

     +       +       =   

Figure 3    Is a Hubble telescope image that was taken through Ha, SII and OIII filters.

Figure 3                  Hubble M16 narrow band image