Subframe Selector

Introduction

After Blink has removed the obviously bad frames, there’s one more powerful step before stacking: PixInsight’s Subframe Selector.

This process uses measurable data — not guesswork — to analyse every subframe and either reject weaker frames or weight the strongest ones so they contribute more to the final image.

Think of Blink as the quick visual check, and Subframe Selector as the detailed quality control.

How to Open Subframe Selector

1. Go to Process
2. Process → Image Inspection → Subframe Selector
3. The tool window will open

Alternative:

• Use Process Explorer → Image Inspection → SubframeSelector

If missing:

• Go to Script → Feature Scripts → Search
• Enable SubframeSelector and restart PixInsight

📸 Menu location screenshot

Why Use It?

Even good-looking frames can hide issues:

• Bloated stars
• Elliptical stars
• High noise
• Transparency variations

Subframe Selector measures:

• FWHM: Star sharpness (lower = better)
• Eccentricity: Star roundness
• SNR: Signal strength
• Background: Sky brightness consistency

Step-by-Step

1. Load subs
   • Use calibrated, Blink-approved frames
2. Check settings
   • Defaults usually fine
   • Add pixel scale if known
3. Measure
   • Click Measure
   • Review generated table

📸 Results table screenshot

Understanding the Data

• FWHM: Lower = sharper
• Eccentricity: ~0.4–0.5 excellent, >0.65 poor
• SNR Weight: Higher = better signal

Next steps:

• Identify outliers
• Reject weakest frames
• Or keep and rely on weighting

📸 Graph example

Weighting Your Frames

1. Enable weights
   • Tick “Output Subframe Weight keyword”
2. Use formula
   • (SNRWeight/(FWHM^2)) * (1 - Eccentricity)
3. Apply
   • Save weights or move files

This prioritises sharp, round, high-signal frames.

📸 Weighting panel screenshot

How Strict Should You Be?

• Large dataset: Be selective
• Small dataset: Use weighting instead
• Golden rule: Quality beats quantity

Where This Fits

Workflow order:

Lights → Calibration → Blink → Subframe Selector → WBPP → Integration

📸 Workflow diagram

Summary

• Second-level quality control after Blink
• Uses real data instead of visual judgement
• Reject or weight frames intelligently
• Directly improves final stacked image quality