Effect of Synthetic Lumenance

It is possible to create an LRGB combine using only RGB data. Some imagers believe that it enhances the detail. The process involves the creation of a synthetic luminance channel.
There are several ways to do this, but one way is to create the RGB image, bring it into Adobe Photoshop, and under the Image menu, select Mode and then Grayscale. This image of NGC434 was taken with 5 minute subframes binned 1x1. I took 12 luminance images and 6 subframes for each of the RGB. This gave a total exposure of L:R:G:B = 60:30:30:30 minutes. The final result although decent appeared to need additional exposure time; however the weather was not cooperative. I got the idea to see what creating a synthetic luminance image and combining it with the real luminance data would result in. While it was not successful because it washed out the color, I decided to look at it more closely and studied the nature of this type of image. As much as is humanly possible I tried to process comparative images identically and end up with similar histograms. I can't help but wonder how it would look if I had gathered 60 minute each of the 3 color channels and added them. If the clouds ever go away I may try this.

LRBG Image

SynthL+LRGB Image

SynthLRGB

True Luminance

Synthetic Luminance

This is the original LRGB
image. While noisy, I was
able to maintain good color
saturation and detail.

When I added the synthetic
luminance data the red and
blue became washed out. I
also saw more noise.

While the color saturation was
better with only the synthetic L
data, the noise prevented me
from pushing the luminance far enough. There are also color gradients from some channels.

This original luminance data
shows, in comparison to the
synthetic data to the right why
there were problems using the
synthetic data.

The synthetic luminance data
has too much noise. This is to
be expected because there is
only 30 minutes of luminance
data left after combining the
3 channels.