Brightness blows up when adding L to RGB composite

I’m an astrophotography newbie and I’m using Siril for Linux to process Slooh FITS images.

With the RGB compositing tool I combine the R, G, B, and L channels of deep sky images to produce color versions. But, when I load the L channel, the brightness of the background sky blows up making it hard to adjust it back to acceptable levels.

Here’s my workflow. In Siril I select the Image Processing > RGB Compositing... option. Next, I load the R, G, and B channels. Finally, I load the luminance channel. As soon as I do the latter, the background sky turns white in all channels as well as the RGB composite and the stars are barely visible.

The screenshot shows what Siril looks like at this point with a set of Slooh RGBL FITS images of NGC 6522. Loading the L channel of planetary RGB composites doesn’t have the same effect on the brightness and the data are still reliable.

When the issue happens, the minimum value of the ☰ > Image Information > Statistics table is 0 and the maximum 65535 for all channels, so there’s not much data I can use. I try to drag the sliders and change the scaling (e.g. Asinh) but adjusting is difficult without the guidance the data provide.

Any advice for keeping brightness and scaling under control when creating RGBL composites?

I’m using Siril in the Crostini Linux container of Chrome OS Stable 90.0.4430.218. The experimental Appimage is easy and convenient to install and the app works well.

Put your visualization sliders to 0 and 65535 please.

Here’s Siril after single-star registration and setting the sliders to 0 and 65535, with Linear:

And Asinh:

With Asinh I can make the sky darker by raising the minimum to, say 1000. Much more manageable.

This is just visualisation. That does not alter pixels.
So whatever you chose that does not change your image.

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Thanks. At some point I need to adjust the pixels so that the result looks good, which is something I’m still struggling with.

I would advice to have a look at our tutorial: Siril - Tutorial for a complete processing of astronomical images with Siril and its scripts

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