SPCC, Masks, and the Art of Mixing Light

Introduction

With your cleaned and noise-controlled master light ready, you are entering one of the most rewarding stages of the workflow: bringing out the true colours of the cosmos. This is where science meets artistry. The goal is to calibrate colours accurately, separate stars if needed, and use masks and curves to craft a balanced, vivid image.

At this stage, your image should already be stretched to non-linear space from the previous step

💡 Save your project now. Work incrementally, duplicating key stages so you can roll back if needed.

1️⃣ Image Solving

Before any colour calibration, the image must be plate-solved so PixInsight knows where each star is in the sky.

• Go to: Script → Astrometry → ImageSolver
• Enter your approximate RA/DEC or let it query online databases automatically.
• Run the solver. You should see a success message with field coordinates.

📷 [Placeholder: ImageSolver settings screenshot]

2️⃣ SPCC – Spectrophotometric Colour Calibration

Where: Process → ColorCalibration → SPCC

• White Reference: Leave default (Average Spiral Galaxy).
• Background Reference: Use a preview in a neutral background region with no stars or nebulae.
• Structure Detection: Keep defaults for now.
• Run SPCC. The image will shift to scientifically calibrated colours based on star spectra.

📷 [Placeholder: SPCC before/after]

3️⃣ Star Separation with StarXterminator

Where: Process → StarXterminator

• Generate both a starless image and a stars-only layer.
• Work on nebula and stars independently to avoid oversaturating stars while enhancing the nebula.

💡 Why: This allows more aggressive colour and contrast work on the nebula while preserving natural star colours.

📷 [Placeholder: StarXterminator interface]

4️⃣ Range Masks – Protect What Matters

Masks let you control which parts of the image are affected.

Where: Process → MaskGeneration → RangeSelection

• Use the Range slider to isolate the nebula or background.
• Apply the mask (drag to target window). Protected areas show in red.
• Invert mask: Mask → Invert Mask (useful for switching between protecting stars vs nebula).
• When finished, remove mask: Mask → Remove Mask.

📷 [Placeholder: RangeSelection interface & applied mask]

💡 Tip: Use masks with curves and LHE to avoid affecting the entire frame at once.

5️⃣ CurvesTransformation – The Alchemy Lab

Where: Process → IntensityTransformations → CurvesTransformation

The Curves toolbar is your “alchemy lab” for colour mixing.

🔴 R / 🟢 G / 🔵 B

Adjust individual colour channels to balance or enhance emission lines.

• Fix dominant channels.
• Emphasise OIII regions by boosting blue.
• Subtle changes preserve natural gradients.

🌈 RGB/K

Controls overall brightness and contrast.

• Gentle S-curves increase dynamic range.
• Use before fine-tuning individual channels.

🌓 L (Lightness)

Boost structure without changing colour.

• Ideal for bringing out faint nebula detail.

🟨 a / 🟦 b (Lab Space)

Green–magenta and blue–yellow axes.

• Correct colour casts quickly.
• Push emission colours artistically.

🎨 c (Chrominance) & 🟫 S (Saturation)

• c: Global saturation intensity.
• S: Pure saturation levels. Use with masks to boost nebula without overcooking stars.

🟪 H (Hue)

Rotate hues to fine-tune star temperatures or nebula tones.

📷 [Placeholder: CurvesTransformation interface]

🌌 Gas & Colour Sidebar

Nebulae and galaxies display colour based on the gases they contain:

• H-alpha: Deep red
• OIII: Teal/cyan
• SII: Crimson
  Stars vary in colour based on temperature:
• Blue-white: Hot, young stars
• Yellow/orange: Cooler, older stars

💡 Art vs Science: You can aim for natural colours using these references or lean into artistic interpretation. Look up NASA/ESA/Hubble or well-calibrated datasets to see reference palettes.

6️⃣ LocalHistogramEqualization (LHE) – Two-Pass Contrast Boost

Where: Process → All Processes → LocalHistogramEqualization

Enhances local contrast and reveals faint structures.

• Coarse Pass:
  • Kernel Radius: 150–200
  • Contrast Limit: ~1.5
  • Apply to starless image or via mask.
• Fine Pass:
  • Kernel Radius: 50–64
  • Contrast Limit: ~1.3
  • Accentuates small filaments.

📷 [Placeholder: LHE before/after]

7️⃣ Recombining Stars and Nebula

Where: Process → PixelMath

• Add the star layer back onto the starless image.
• Use starless + stars as the PixelMath expression.

💡 If recombined image looks flat:

• Check star layer brightness – reduce intensity before merging.
• Apply gentle curves to match luminance between layers.

✅ Quick-Reference Checklist

• Save project and duplicate key stages.
• Run ImageSolver.
• Apply SPCC with a neutral background preview.
• Separate stars with StarXterminator.
• Create and use Range Masks for controlled edits.
• Use CurvesTransformation for colour and contrast shaping.
• Apply two-pass LHE for balanced local contrast.
• Recombine stars and nebula with PixelMath.
• Save final project and export as a high-quality TIFF or XISF.