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What recipes do you use?

Pleiades Astrophoto PixInsight Processing LRGB Luminance Monochrome RGB Intermediate
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Aris Pope avatar
Aris Pope avatar

I've been imaging OSC for the past 3 years and recently switched to monochrome. I use Pixinsight.

Combining L with RGB reminds me of apple pie. And there's a million recipes for apple pie. What recipes do you use for combining L with RGB?

I've been using the image blend but still learning. I would like to just use the RGB stars and not the Luminance stars(different/longer exposures). Should I add the Luminance and RGB together with stars? Then remove the stars and add the stars from the RGB back in?

Thanks

Aris

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andrea tasselli avatar
andrea tasselli avatar
Aris Pope:
I've been using the image blend but still learning. I would like to just use the RGB stars and not the Luminance stars(different/longer exposures). Should I add the Luminance and RGB together with stars? Then remove the stars and add the stars from the RGB back in?


Yes, if you can. Sometimes you can't.
Read noise Astrophotography avatar
Read noise Astrophotography avatar

You’re right, LRGB is like apple pie …… lots of recipes. A couple that work reliably in PI, especially if you want to keep RGB stars and not the bloated L stars:

Workflow 1 Classic LRGB with RGB stars

  1. Process RGB to taste → gradient removal, color calibration, noise reduction. Don’t push saturation yet.

  2. Process L → DBE, deconvolution if needed, noise reduction, stretch for detail.

  3. Star removal on both with StarXTerminator (or Starnet).

  4. LRGBCombination (PI tool):

    • Use starless L + starless RGB.

    • Adjust sliders (start with L = 0.5, Chrominance Noise Reduction = on).

  5. Add RGB stars back → PixelMath:

    starless_LRGB + stars_RGB

    That keeps color-accurate stars while L drives nebula/galaxy detail.

Workflow 2 – Synthetic L / SuperL

If L stars are way sharper than RGB stars and you don’t want mismatch:

  1. Combine R+G+B into a synthetic L (ChannelCombination then ExtractL).

  2. Blend synthetic L with real L → PixelMath (weighted average, e.g. 0.6L_real + 0.4L_synth).

  3. Use that as your luminance layer.

  4. Same as Workflow 1: remove stars, combine, then restore RGB stars.

Workflow 3 – Direct blend (no LRGB tool)

Sometimes easier:

  1. Convert RGB to Lab color space.

  2. Replace L channel with your processed L frame.

    • In PI: ChannelCombination in Lab mode, set L = your L master, a & b from RGB.

  3. StarX both, swap in RGB stars at the end.

Tips

  • Don’t force 100% L. Often 50–70% L mixed with RGB preserves color saturation better.

  • Color first, detail second: nail your RGB color balance before bringing in L, or you’ll chase odd hues later.

  • If L stars look sharper and you want them, you can blend L+RGB stars 50/50 with PixelMath, but that’s taste.

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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar

I simply use the LRGB Combination tool in Pixinsight, but beware that the images need to be stretched first. It does not work with linear images. I simply tweak to taste as I go along. Or, you could do it in Photoshop by adding L as a layer and choose “luminosity blend mode” which preserves hue/saturation while adding the lum. Basically the same thing.

I would remove all the stars in a linear state after you’ve done the necessary processing like color calibration, then just chuck the lum stars and stretch the RGB stars to taste. Add them back after all background processing has been done as a final step.

You should however not be using “starless+stars” as a formula in pixelmath to add stars back into the image. The stars should always be screen-blended back in a non-linear state. This is easy enough in Photoshop by choosing “screen” as blend mode.

In Pixinsight you should use the correct formula, there are several ones that does the same thing, but this is what I’ve saved as my process icon: ~((~starless)*(~stars))

Alternatively; combine(starless, stars, op_screen())

This will do the exact same thing that Photoshop does with screen blend mode. Using this will protect highlights and reduce the chance of artefacts, to mention a few advantages.

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Craig Bernier avatar
Craig Bernier avatar

Adam Block and Mike Cranfield’s Image Blend utility is a real game changer for this (imo). It offers quite a massive amount of flexibility in the blends, both with SHO to RGB to L and so on. If you don’t have it, try it out. There’s more than a few YouTube tutorials out there already and if you want it straight from the horse’s mouth, his “Infinity” subscription on his site will soon have a full-blown course. Full-disclosure: I don’t get a kick-back for recommending this utility, only better pixinsight processing end results. 😄

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