The techniques below explore the possibilities for producing more accurate colors from narrowband images.  Narrowband images are inherently false color because they do not duplicate the color response of the human eye.  Many false color images can be very beautiful, but sometimes it is desirable to simulate the more realistic colors of an RGB image using narrowband filters.  First, a little background on why narrowband images are usually significantly different in appearance from RGB images and how a closer approximation can be achieved.  Step-by-step techniques follow.

Why Narrowband Images are False Color

The standard set of narrowband filters used by amateur imagers consists of Hydrogen-alpha, Oxygen-III, and Sulfur-II.  These are abbreviated to H-alpha, OIII, and SII.  Other filters sometimes used include Nitrogen-II (NII), Helium-II (He-II) and Hydrogen-beta (H-beta).  Combined in the usual fashion, H-alpha, OIII, and SII filters give an overall reddish color to most emission nebulas.  See the page on Using Clipping Masks to Create Color Images for details on this method.

Above:  Lagoon Nebula combined using standard method outlined in the Using Clipping Masks to Create Color Images section

While the Lagoon Nebula is predominantly red in RGB images, it does not contain the yellow coloration seen above.  This isn't necessarily a bad thing, as this is still a pretty picture.  But there are ways to more accurately simulate the real colors of the nebula.

RGB images work by capturing light from three parts of the visible spectrum, red, green, and blue.  The usual narrowband filters, on the other hand, capture light from only the red and green parts of the spectrum.  Both H-alpha and SII filters transmit red light, and OIII transmits green light.  (Actually, the light from OIII is somewhat blue-green, but more on that later.)  There is no light being captured from the blue portion of the spectrum.  One way to remedy this problem is to use a Hydrogen-beta filter, which passes blue light, instead of the SII filter.  That way all three parts of the spectrum are represented.

The methods below avoid the use of the H-beta filter and show methods for recreating the effect of H-beta either using only the H-alpha and OIII filters, or a similar method incorporating the SII filter as well.

Two Methods for Simulating RGB in Narrowband Images

The first method was devised by Travis Rector, whose work on Hubble Space Telescope images has strongly influenced amateur imaging techniques.  It involves a method for recreating the effect of an H-beta filter while using only two original images -- one through an H-alpha filter, and one through an OIII filter.  This has the added advantage of requiring fewer images to be taken, thereby shortening the required exposure times.

The second method uses a similar technique for reproducing the effect of an H-beta filter but also incorporates SII data for a more accurate representation of the nebulosity.  It also tends to produce a more accurate OIII emission line color than other methods.  This method seems to most closely approximate the colors of a normal RGB image, but with the enhanced detail of a narrowband image.

Method 1

The idea behind this image is that most of the color seen in an RGB image of an emission nebula comes from the dominant lines of Hydrogen-alpha and Hydrogen-beta, and by Oxygen-III light.  H-alpha is red, OIII is mostly green, and H-beta is blue.  Instead of requiring a separate H-beta image, this method uses the existing H-alpha and OIII data to create the effect of an H-beta image.  This method will be referred to here as the simulated H-beta method.

The effect of having H-beta in an image is to tone down the pure red color of the H-alpha emission (which is the dominant emission in most nebulas), turning it into the familiar pinkish or magenta hue of nebulas in RGB pictures.  Since H-alpha and H-beta emissions are both given off by hydrogen gas, they will tend to occur in the same location within a nebula.  On the other hand, OIII and SII data tend to correlate to different structures within the nebula.

Step 1

Begin by creating a mapped-color image as outlined in the section on Using Clipping Masks to Create Color Images.  Normally you would use H-alpha, OIII, and SII for red, green, and blue respectively.  Instead, create the image using H-alpha for red, and OIII for both green and blue.

Above:  Layers window for the mapped-color image using Ha, OIII, and OIII

Step 2

Now, copy the H-alpha layer and paste it on top of all the other layers.  As with the other layers, set the Blend Mode to Screen.  Create another Hue/Saturation adjustment layer and clipping mask as you did for the other layers.  Use this to map the color of the duplicated H-alpha layer as blue.  Then lower the Opacity to around 15%.

Above:  Layers window after mapping the second H-alpha layer to blue and lowering opacity

This has the effect of blending in some of the hydrogen emission as blue, which is the basic equivalent of using an H-beta image.  This creates a pinker color for the nebulosity which more closely approximates the RGB color.

Above:  Lagoon Nebula processed using the simulated H-beta method

Method 2

This method attempts to very closely simulate the real effects of the narrowband emissions in producing RGB colors.  There are many wavelengths which contribute to color in a nebula, and H-alpha, OIII, and H-beta are the primary ones.  This allows the simulated H-beta method to work very well at recreating RGB colors.  However, a somewhat closer approximation can be obtained by incorporating the SII data.  This also allows interesting features which only appear in SII light to be retained.

The theory behind this method, called the simulated RGB method (since it tries to recreate an RGB image from narrowband data), involves using the actual intensity ratio between H-alpha and H-beta light.  The theoretical intensity ratio between H-alpha and H-beta is 2.9:1, meaning H-alpha should be about 3 times brighter than H-beta.  Data from spectral analysis of nebulas confirms this ratio.

By blending the H-alpha data with the OIII as before, a more accurate representation of the true hydrogen color is achieved.  This also makes the OIII color more of a teal hue than pure blue or green, which tends to occur with the standard method and simulated H-beta method, respectively.  The overall appearance at this stage is one of excessive pink or magenta in the nebula.  The SII data is then added back in and mapped as red.  This brings the overall color back in line with the expected RGB result and also adds in the SII data.

Step 1

Begin by creating the mapped-color image as above, with the H-alpha, OIII, and OIII layers as red, green, and blue, respectively.

Above:  The starting Layer window settings

Step 2

Next, copy the H-alpha layer and paste it on top of the blue OIII layer, but under the Hue/Saturation adjustment layer.  Lower the opacity to 34% (which is 1/2.9, the ratio of H-beta to H-alpha).

Above:  H-alpha layer copied on top of the blue OIII layer, with Opacity lowered

Step 3

Add the SII image as another layer on top of everything else and map is as red, setting the Blend Mode to Screen as was done with the other layers.

Above:  SII layer mapped as red

The only problem now is that the extra red layer tends to make the background too red.  The last step is any easy fix to this problem.

Step 4

Create a Layer Mask in the SII layer.  Select and copy the bottom H-alpha layer.  Alt-click on the layer mask to make it the active window.  The image will go white because you are now viewing the layer mask instead of the main image.  Paste the H-alpha image into the layer mask.  Apply a large Gaussian Blur to the mask (about 10-30 pixels, depending on the image size).  Then open the Curves window (Ctrl-M) and adjust the curve to look like the window below.

Above:  Curves window applied to the layer mask

Click on the left window in the SII Layer to return to the main image.

Above:  Layers window with the layer mask applied to the SII layer

The SII now remains applied to the nebula but the background stays black.  The result is very close to the color expected from an RGB image, but with greater detail.

Above:  Simulated RGB narrowband image of the Lagoon Nebula

Comparisons of the Various Methods

Below are a few examples showing the difference between the standard, simulated H-beta, and simulated H-alpha methods of narrowband color combining.  These are strictly a matter of aesthetics and there is no right or wrong way to combine the images.  Other adjustments can be made to tweak each layer individually if desired, and more than three or four layers and colors can be used.  You can map NII to purple and He-II to turquoise for all anyone cares, as long as you like the results. 

That being said, note the significant addition of data in the red portions of the Veil Nebula image when the SII image is incorporated.  The processing on all the image below was identical expect for the color combining method.

Above:  Veil Nebula.  From left to right, standard, simulated H-beta, and simulated RGB methods

Above:  Swan Nebula.  From left to right, standard, simulated H-beta, and simulated RGB methods

Above:  North America Nebula.  From left to right, standard, simulated H-beta, and simulated RGB methods

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