Processing Narrowband Images

The standard processing methods such as stretching, filtering, etc., still apply to narrowband images the same as they do to regular images.  The primary factor in processing narrowband images is in deciding which color channels to assign to each filter and in properly weighting the channels to achieve a pleasing color (which is somewhat subjective due to the fact that these are false color images).  The method below assumes the use of a CCD image processing program like MaxIm DL.  However, a more sophisticated method using layer clipping masks in Adobe Photoshop is outlined in the Software Instructions section (Layer Clipping Mask method), as are two new methods for improved narrowband color.

Color Combining

For a typical emission nebula, you can achieve an approximation of true color by combining the colors using the HOS method (H-alpha = red, OIII = green, SII = blue).  This is due to the fact that in true color, emission nebulae are primarily red.  By assigning the H-alpha image -- which should be the brightest image -- to the red channel, the nebula will appear mostly red.  Each channel is normally weighted equally (1:1:1), unless the CCD is particularly deficient in one part of the spectrum.  For example, the ST-2000XM is only half as sensitive in red as in green and blue, so it might be weighted 2:1:1.  Either way, the balance is normally the same as you would use for your RGB images.

Many images look spectacular using the Hubble Color Mix (SHO method).  In this method, the H-alpha image is assigned to the green channel, so green will be the dominant color.  With most CCDs, a 1:1:1 balance will usually be too green.  A 2:1:2 balance (or 1:0.5:1) balance is usually preferable.  Again, a camera like the ST-2000XM, which is less sensitive to H-alpha already, will probably look fine using a 1:1:1 balance with the SHO method.

Certain nebulae that are not as bright in H-alpha, such as planetary nebulae, can look great in several different color combinations.  The HOO method, using OIII for both blue and green channels, is particularly good as it simulates the standard true color view fairly well.  This method works well on certain emission nebulae as well.

Examples

Above:  M27 using HOS at 1:1:1

Above:  M27 using SHO at 1:0.5:1

Above:  M27 using HOO at 1:1:1.  This provides a close approximation to true color.

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