Emission Colors 1

Emission Colors

An overview

Broadband light sources (like stars) can be accurately represented by combining the wideband RGB channels. When you do this, the colors of the line-spectra sources (emission nebula) will be incorrect. Figure 1 is an example. This is representative of most Veil Nebula pictures.

Line spectra sources can be accurately represented by properly combining the narrowband image frames. "Combining" means adding weighted amounts of each channel to produce the three output primaries. The correct weighting for line emissions causes the broadband sources to be wrong (the stars will be blue-green), figure 2 below.

Figure 1, wideband RGB frames corrected for broadband sources

Figure 2, narrowband "a3b" frames corrected for line emission sources

If you don't have the narrowband filter images, you can still obtain the correct colors for the line emissions from the usual wideband channels. A different weighting matrix is used, and the nebula signal is weaker than when narrowband channels are available (because exposure time is shorter for the wideband filters). The stars are still blue-green (figure 3 below), but the nebula colors are right.

The final combination is trying to get accurate star colors out of the narrowband channel data. In principle, this should work. In practice however, the nature of the filters, the levels of noise, and other factors, prevent this combination from working (figure 4 below).

Figure 3, wideband RGB frames corrected for emission colors

Figure 4, narrowband frames (unsuccessfully) corrected for broadband illumniation sources

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An overview Broadband light sources (like stars) can be accurately represented by combining the wideband RGB channels. When you do this, the colors of the line-spectra sources (emission nebula) will be incorrect. Figure 1 is an example. This is representative of most Veil Nebula pictures.	Line spectra sources can be accurately represented by properly combining the narrowband image frames. "Combining" means adding weighted amounts of each channel to produce the three output primaries. The correct weighting for line emissions causes the broadband sources to be wrong (the stars will be blue-green), figure 2 below.
Figure 1, wideband RGB frames corrected for broadband sources	Figure 2, narrowband "a3b" frames corrected for line emission sources
If you don't have the narrowband filter images, you can still obtain the correct colors for the line emissions from the usual wideband channels. A different weighting matrix is used, and the nebula signal is weaker than when narrowband channels are available (because exposure time is shorter for the wideband filters). The stars are still blue-green (figure 3 below), but the nebula colors are right.	The final combination is trying to get accurate star colors out of the narrowband channel data. In principle, this should work. In practice however, the nature of the filters, the levels of noise, and other factors, prevent this combination from working (figure 4 below).
Figure 3, wideband RGB frames corrected for emission colors	Figure 4, narrowband frames (unsuccessfully) corrected for broadband illumniation sources