Narrowband post-processing in Affinity Photo Software

According to ChatGPT…..

To do **narrowband post-processing** in **Affinity Photo**, especially for astrophotography (e.g., H-alpha, OIII, SII), you can follow a structured workflow. The idea is to map each narrowband channel into RGB space and enhance contrast, detail, and color. Here’s a step-by-step guide:

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### 🧰 Prerequisites

Make sure you have:

* Stacked and calibrated FITS or TIFF files for each filter (e.g., H-alpha, OIII, SII).

* Optionally converted them to 16-bit TIFFs (Affinity Photo doesn’t support FITS natively, but TIFFs from DeepSkyStacker, Siril, or PixInsight work well).

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## 🌈 STEP 1: Load and Assign Channels

1. **Open each file** in Affinity Photo:

   * File → Open → Select H-alpha, OIII, and SII files.

2. **Create a new RGB image**:

   * File → New → Set desired dimensions and 16-bit RGB.

   * This will be your master composite.

3. **Copy-paste each channel** into the new image:

   * Go to each narrowband image.

   * Select → Select All → Edit → Copy.

   * Go to your RGB master:

     * Channels panel → Select *Red*, *Green*, or *Blue*.

     * Edit → Paste (or drag from Layers and rasterize into channel).

     * Common combinations:

       * **Hubble Palette (SHO)**: SII = Red, H-alpha = Green, OIII = Blue.

       * **Bi-color (HOO)**: H-alpha = Red, OIII = Green & Blue.

       * **Natural-ish (HVO)**: H-alpha = Red, OIII = Green, OIII+H-alpha blend = Blue.

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## 🎨 STEP 2: Tone and Color Adjustments

### A. **Stretch the image**

1. Use **Levels** or **Curves** to stretch the histogram:

   * Layer → New Adjustment Layer → Levels or Curves.

   * Adjust black and white points to bring out detail.

### B. **Enhance Color**

* Layer → New Adjustment Layer → HSL.

* Use sliders to adjust hue, saturation, and lightness per color channel.

* For Hubble palette:

  * Shift green (H-alpha) toward gold.

  * Push blues (OIII) toward teal.

  * Boost reds (SII) for magentas.

### C. **Reduce color noise**

* Layer → New Live Filter Layer → Denoise.

* Adjust Luminance and Color sliders carefully.

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## 🔬 STEP 3: Sharpening and Contrast

### A. **Local contrast / detail**

* Use Unsharp Mask or Clarity filter:

  * Layer → New Live Filter Layer → Clarity / Unsharp Mask.

  * Use sparingly to avoid halos.

### B. **High Pass for fine detail**

1. Duplicate the composite layer.

2. Apply Filters → Other → High Pass.

3. Set the layer blend mode to **Overlay** or **Soft Light**.

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## 🌌 STEP 4: Star Reduction or Separation (optional)

Affinity doesn’t have star masks, but you can:

* Select stars via **Select → Select Sampled Color**.

* Feather the selection.

* Apply a Gaussian Blur or clone brush on a duplicated layer.

* Or use plugins or do star removal in **Starnet++**, then recombine in Affinity.

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## 🧼 STEP 5: Final Touches

* Crop, rotate, and add any text/labels.

* Export as 16-bit TIFF or JPEG (depending on use).

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## ✅ Summary Workflow

| Step | Action                                 |

| —- | ————————————– |

| 1    | Load narrowband images (SII, Hα, OIII) |

| 2    | Create RGB composite using Channels    |

| 3    | Stretch via Levels or Curves           |

| 4    | Adjust color using HSL and Curves      |

| 5    | Enhance details (Clarity, High Pass)   |

| 6    | Star tweaks or separation (optional)   |

| 7    | Export as TIFF or JPEG                 |

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