The best of both wide- and narrowband color-correct images (figures 1 and 2 of the previous page) can be combined by making a mask that separates the stars from the nebula and then mixing the two images according to the mask. It is tempting to try and select pixels based on color information, but it turns out to be more reliable to make a spatial mask (figure 5). A median filter of the right size can identify the stars. This "median mask" needs to be smoothed slightly and amplitude scaled so that it can be used for interpolation. We can then use it to blend the right pixels from each source image and get the final desired result: a colorimetrically correct nebula in the presence of correctly tinted stars (figure 6).

Figure 5, median filter generated mask identifies the stars

Figure 6, combined color image, correct for both stars and nebula.

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Copyright 2002-Feb-03 Thor Olson

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