Combining the Colors

Narrowband filters do not attempt to replicate the spectral sensitivity of the human eye.  Therefore, color images created from these filters are called false color images.  Typically, three filters are used and each is assigned to one channel of an RGB image.  One filter becomes the red part of an image, one becomes the green part, and the third is the blue part.  Once combined, each color represents a particular wavelength of light and hence a particular element in the gas cloud.  In addition to being a pretty picture, a narrowband image is also scientifically interesting (which is why these types of filters are used on the Hubble Space Telescope and other professional instruments).

The most commonly used filters are H-alpha, OIII, and SII.  The order in which the narrowband filters are combined in an RGB image is arbitrary.  The most common two methods are listed below.  Keep in mind that H-alpha is almost always the dominant emission from star-forming regions, so whatever channel you assign H-alpha to will be the primary color of the final image.

HOS

H-alpha = Red
OIII = Green
SII = Blue

In this method, the dominant emission line (H-alpha) is assigned to the red channel.  Since this emission line is what normally contributes to the red color in normal RGB-filtered images on nebulae, this method leads to an approximation of true color, i.e., emission nebulae look predominantly reddish.  However, in this method, the blue channel is coming from SII, which is actually in the red part of the spectrum -- redder than H-alpha, in fact -- and so there is not really any part of the blue spectrum involved in the image.

Above:  Lagoon Nebula in HOS.  Red is from H-alpha emission, magenta from a combination of H-alpha and SII, and yellow from a combination of H-alpha and OIII.

SHO ("Hubble" Colors)

SII = Red
H-alpha = Green
OIII = Blue

At first, this doesn't seem like a logical combination, since we think of OIII as green light and H-alpha as red.  But, this method actually puts the filters in order from bluest to reddest.  Again, there is no blue part of the spectrum involved, but OIII is closest to the blue end of the spectrum, SII is closest to the red end, and H-alpha is in between.  This combination is used in some of the famous Hubble images such as the "Pillars of Creation".  Since H-alpha is assigned to the green channel, images combined this way tend to look predominantly green.

Above:  Swan Nebula in SHO.  Green is from H-alpha, turquoise from a combination of H-alpha and OIII, and yellow from a combination of H-alpha and SII.

Other Methods

HaOHb

H-alpha = Red
OIII = Green
H-beta = Blue

The basis for this combination is that each filter represents one part of the visual spectrum.  H-beta is actually in the blue part of the spectrum, OIII in the green, and H-alpha in the red.  SII or NII could easily be substituted for the H-alpha, but H-alpha is still the dominant emission in most nebulae, so its use is usually recommended.

SNH

SII = Red
NII = Green
H-alpha = Blue

This method uses all three of the emission lines in the red part of the spectrum.  It leaves the lines in order from bluest to reddest, like the HOS method.  Since H-alpha is the blue channel, and since NII is second more prominent line, the images tends to be more bluish-green.  Another possibility is to use the same filters but make H-alpha red which will give a reddish-yellow cast to the nebula.

HOO

H-alpha = Red
OIII = Green
OIII = Blue

There are a couple advantages to this method.  First, it requires 1/3 fewer images, as only two filters are used.  The OIII image is used for both the green and blue channels of the final RGB image.  Also, many objects have very little emission outside of these two lines, so there often is not much advantage to spending extra time capturing those other emission lines.  This method works particularly well on the Veil Nebula and is a good choice for planetary nebula as well, since their emission is mostly from excited oxygen.

4+ Channel Color

Programs such as Photoshop and MaxIm DL allow you to combine more than three colors, so it is possible to combine four or five or more different filters.  This is also a technique used in many Hubble pictures.  Typically you would start by assigning three channels to red, green, and blue, as usual, and then progress to the secondary colors cyan, magenta, and yellow.  But again, this is an arbitrary system, so do whatever looks best to your own eye. 

This technique can also be used to assign colors other than red, green, and blue to the standard three filter set.  For example, OIII is actually in the blue-green part of the spectrum, so it could be assigned a turquoise color.  Or H-alpha could be given red while SII is assigned a dark brick red closer to its actual spectral color (although the results may or may not be desirable!). 

Above:  A color combination in the Lagoon Nebula you've never seen before...and probably don't want to see again!

Really any combination is feasible, and again the combinations are arbitrary, so do whatever strikes your fancy!  The image below uses the HOS method but adds a blue-filtered image to capture the reflection nebula above the main Orion Nebula which is invisible through narrowband filters.

Above:  Orion Nebula through H-alpha, OIII, SII, and blue filters.  Image was taken from a suburban backyard and with a first quarter moon out.  This demonstrates a couple of the advantages of narrowband imaging.

Next, Taking Narrowband Images...

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