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Ashen Chathuranga
Ashen Chathuranga

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Astrophotography Stacking Guide

#linux #mobile #opensource #tutorial

The Complete Guide to Mobile Astrophotography Stacking

Device: Motorola Edge 60 Fusion | OS: Fedora Linux | Software: Siril

Stacking combines multiple long exposures to reduce digital noise and reveal faint details of the Milky Way that a single phone camera shot cannot capture. This guide takes you from field capture to the final processed masterpiece.

PHASE 1: Capturing the Night Sky (In the Field)

1. Set Up Your Gear

  • Tripod: Mount your phone on a sturdy tripod. Any movement ruins the alignment.
  • Trigger: Use a Bluetooth remote or a 2-second delay timer so tapping the screen doesn't shake the phone.
  • Intervalometer: Use an intervalometer app (if available) to automatically trigger the shutter repeatedly.

2. Configure Pro Mode

Open your default camera app, switch to Pro Mode, and apply these exact settings for your 24mm main lens:

  • Format: RAW (DNG). Crucial: JPEGs cannot be stacked properly.
  • Focus: Manual, set all the way to Infinity (∞).
  • Shutter Speed: 16 seconds. (The "500 Rule" limits this lens to ~20 seconds before stars trail; 16 seconds guarantees sharp pinpoint stars).
  • ISO: 1600 to 3200 (Start at 1600; adjust based on light pollution).
  • White Balance (WB): Manual, around 4000K (makes the sky a neutral blue/grey).

3. Capture "Light" Frames

  • Point your camera at the target (e.g., the Milky Way core).
  • Take a continuous sequence of 40 to 60 photos without moving the tripod.

4. Capture "Dark" Frames

  • Immediately after your last light frame, completely cover the phone's camera lenses (use a thick cloth or lay it flat on a dark surface). No light can enter.
  • Keep the exact same ISO and Shutter Speed (16s).
  • Take 15 to 20 photos.
  • Purpose: Maps the thermal heat noise generated by the sensor during a 16-second exposure.

PHASE 2: Capturing Calibration Frames (At Home)

5. Capture "Bias" Frames

You can do this inside your house at any time.

  • Keep the lenses completely covered (pitch black).
  • Set the ISO to the same value used for your Lights/Darks.
  • Change the Shutter Speed to the fastest possible setting (e.g., 1/4000s or 1/8000s).
  • Take 20 photos.
  • Purpose: Maps the instantaneous electronic read noise of the camera sensor.

PHASE 3: Preparing the Data (On Fedora)

6. Install Siril

Open your terminal and install Siril from the official repositories:

sudo dnf install siril

7. Organize Your Folders

Siril automated scripts require a strict, specific folder structure to work.

  1. Create a main project folder (e.g., Desktop/MilkyWay).
  2. Inside it, create three sub-folders exactly named (all lowercase, plural):
    • lights (Put your 40-60 star DNGs here)
    • darks (Put your 16s dark DNGs here)
    • biases (Put your fast-shutter bias DNGs here)

8. Add the Missing Script

  1. Open Siril.
  2. Click the "Scripts" menu at the top.
  3. Click "Get Scripts".
  4. Find and install the script named OSC_Preprocessing_WithoutFlat (OSC = One Shot Color).

PHASE 4: Stacking in Siril

9. Run the Automation

  1. Click the "Home" icon (Change working directory) in the top left of Siril. Select your main MilkyWay folder.
  2. Go to Scripts -> Siril Script Files -> OSC_Preprocessing_WithoutFlat.ssf.
  3. Let the script run. It will calibrate, align, and stack your images. This takes a few minutes.
  4. Wait for the green text in the console: "Execution of the script is successful."

PHASE 5: Post-Processing the Masterpiece

10. Reveal the Data

  1. Click the Open button (top left) and open result.fit from your MilkyWay folder.
  2. The screen will be dark. Look at the bottom-center of the Siril window. Change the dropdown from Linear to AutoStretch. Your stars will appear!

11. Crop the Artifacts

Because the Earth rotated, Siril had to rotate your images to align the stars, leaving jagged black borders.

  1. Click and drag a box over the good part of your image, excluding all black edges.
  2. Right-click inside the box and select Crop.

12. Fix the Colors (Remove the Green)

Stacking RAWs makes the image very green. Let's neutralize it.

  1. Draw a small selection box over a dark, empty part of the sky (avoid bright stars or the Milky Way band).
  2. Go to Image Processing (top menu) -> Color Calibration -> Color Calibration.
  3. In the "Background Reference" section, click Use current selection, then click Background Neutralization. The green cast will vanish! Close the window.
  4. Go to Image Processing -> Remove Green Noise. Click Apply and close.

13. Permanently Stretch the Image

AutoStretch is just a preview. If you save now, it will be pitch black.

  1. Change the bottom-center dropdown from AutoStretch back to Linear. (The image goes dark again).
  2. Go to Image Processing -> Histogram Transformation.
  3. Click the Gear Icon in that window to apply the AutoStretch settings permanently.
  4. Click Apply and close the window.

14. Export and Share!

  1. Click the Save As icon (the arrow pointing down to a hard drive near the top right).
  2. Change the file type extension from .fit to JPEG or TIFF.
  3. Name your file (e.g., Final_MilkyWay.jpg) and click Save.

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