In LRGB imaging, the luminance image controls how strongly each of the color channels appears in the image. If you use a red-filtered image as the luminance, then red objects will appear disproportionately bright in the resulting color image. Likewise, replacing the luminance image with the blue-filtered image will enhance the brightness of blue objects in the image. Used with discretion, this technique is a powerful tool because enhancing the brightness of red or blue features in the image does not alter the overall image color balance, but it does permit emphasizing image features.
A useful variation on this technique is to replace the luminance channel with a deep image taken through an Ha filter. This strongly enhances the presence of low-excitation red nebulosity. In high-excitation objects, such as supernova remnants, the OIII at 500.7 nm may be dominant in terms of total energy, but because this wavelength often falls into a gap between the transmittance curves of the green and blue filters, this astrophysically important emission will appear too dark. This can be corrected by using an OIII image as luminance.
However, replacing the luminance channel with a non-luminance image is an all-or-nothing proposition. Instead, it is possible to blend multiple luminance channels to allow fine control over the resulting image. If a normal luminance image and an Ha image can be blended in any proportion, it would be easy to create color images having enhanced nebular brightness without dimming the hot blue stars that illuminate the glowing gas cloud.
Similarly, the color representation of an object that emits strongly in both Ha . and OIII can be "fine tuned" to show its red- and teal-colored components equally well, rather than strongly favoring one or the other.
• Tip: AIP4 Win's Join Colors Tool accepts two simultaneous luminance channels. The different luminance images can be the same luminance image differently processed (for example, a linear processing and an exponentially histogram-shaped one), a normal luminance and the color channel of your choice, a normal luminance and one Ha, or any other combination limited only by the user's imagination.
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