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Mosaic Workflow

A mosaic combines multiple overlapping panels into one larger field. The key is consistency: each panel should be calibrated, integrated, background-corrected, stretched, and color-balanced in a way that does not create visible seams.

Use this workflow for:

  • Wide nebulae, Milky Way regions, and galaxy fields that do not fit one frame.
  • Multi-panel LRGB, OSC, narrowband, or mixed-filter projects.
  • Datasets where each panel has overlapping stars with neighboring panels.

Pipeline

The release guide does not include a diagram of this workflow. AfterNight does not yet expose a verified end-to-end native mosaic pipeline, and no approved overlapping-panel dataset has passed the workflow gate. The sections below are retained as planning notes; they do not describe a verified workflow.

1. Plan And Group Panels

A mosaic starts before processing. Keep panel identity visible in folders, filenames, tags, or Image Integration groups.

Requirement Why it matters
Overlap between panels Gives registration/blending enough shared stars
Similar exposure depth Avoids one panel becoming noisy or dominant
Consistent filter/session naming Keeps calibration and composition manageable
Stable rotation and scale Reduces resampling artifacts
Flats per panel/session when needed Prevents panel-specific vignetting seams

If panels were captured across nights, treat session differences as real processing variables, not just folder names.

2. Integrate Each Panel

Use Image Integration for each panel or for a grouped project that preserves panel metadata. Keep calibration, registration, normalization, and stacking settings consistent unless one panel has a clear reason to differ.

For each panel:

  1. Calibrate with matching masters.
  2. Register lights within the panel.
  3. Stack to a linear panel master.
  4. Inspect the report and rejection output.
  5. Save the linear master before later edits.

Do not aggressively crop individual panels before alignment; keep overlap available.

3. Solve And Align Panels

If the panel masters have WCS metadata, use Plate Solving to verify it. SIP distortion correction is handled by the engine when FITS metadata contains the needed coefficients, and the registration/alignment pages document that behavior.

Use Image Registration when panels need a common geometry. For large mosaics:

  • Use a good reference panel or solved reference.
  • Prefer alignment modes that preserve the full field if you need every panel.
  • Use intersection/crop later only after confirming the mosaic coverage.
  • Save registered panels separately from originals.

4. Normalize Backgrounds And Gradients

Panel seams often come from sky background differences rather than registration failure. Work while the panels are still linear:

Avoid fitting away real nebula gradients. In very large fields, true sky structure can span panels.

5. Compose And Crop

After panels share geometry and background scale, compose the mosaic. Depending on your current build and dataset, this may involve a dedicated composition step outside AfterNight or a manual panel-blend workflow using registered masters and blend/geometry tools.

After composition:

  • Use Crop to remove blank or low-coverage edges.
  • Use Resize only after composition and inspection.
  • Keep a linear mosaic master before any permanent stretch.

6. Color, Stretch, And Finish

For broadband mosaics, follow the color and stretch guidance in Broadband Imaging. For narrowband mosaics, follow Narrowband Imaging.

Common final sequence:

  1. Background extraction or final seam cleanup.
  2. Spectrophotometric Color Calibration, Photometric Color Calibration, or composition-specific color balancing.
  3. Generalized Hyperbolic Transformation or Histogram Transformation.
  4. Denoise, local contrast, star treatment, and final saturation.
  5. Export in a master format and a display format.

Seam Checklist

Before calling the mosaic finished, inspect:

Area What to look for
Background Panel-to-panel brightness steps or color tint shifts
Stars Doubled stars or stretched stars along seams
Noise One panel visibly noisier after stretch
Color Filter/session mismatch across overlap
Edges Blank canvas, low-coverage corners, or resampling artifacts

Troubleshooting

Symptom Likely cause Fix
Panels do not align Too little overlap, weak WCS, different scale/rotation, or bad reference Solve panels, choose a stronger reference, or align externally before returning
Visible seams after stretch Background or normalization mismatch Rework background extraction and linear matching before stretch
One panel has different star size Seeing, focus, filter, or resampling mismatch Process that panel separately, reduce stars, or use masks for local correction
Mosaic crop removes too much field Intersection-style alignment or early crop was too aggressive Rebuild from full-panel masters and crop only after composition