Mastering Blink: Screening Subframes for Quality Stacking

Introduction

Even the best-calibrated data can be ruined by poor-quality subframes. Focus shifts, guiding errors, thin clouds, or vibrations can all creep into a session, and if they’re not caught early, they’ll degrade your final stacked image. The Blink process is where you quickly review every calibrated subframe, remove the weak ones, and keep only the best for stacking.

Why Blink Matters

Stacking relies on combining multiple exposures to boost signal and reduce noise. If bad frames are included, they don’t just fail to add value – they actually harm the result by introducing blur, artefacts, and inconsistent background levels. A single soft or distorted subframe can drag down the quality of the entire stack.

By taking a few minutes to review your subs, you ensure that only the sharpest, cleanest data contributes to the master image.

📸 [Placeholder: Screenshot showing a set of subs in Blink]

What to Look For When Reviewing Subs

• Star Sharpness and Focus
  Stars should appear round and tight. Watch for soft focus or “donut” shapes caused by defocus.
• Tracking Errors or Star Trailing
  Trails indicate guiding or mount issues. Slight elongation may still stack, but heavy trailing will ruin the image.
• Frame Shifts or Rotation
  Compare stars across the field. Any frame with a sudden shift or rotation is a candidate for rejection.
• Transparency Changes
  Look out for frames dimmed by passing thin cloud or haze. Uneven background gradients are a tell-tale sign.
• Satellite and Aircraft Trails
  Some can be fixed during stacking, but strong trails across key areas are often best rejected.
• Wind or Vibration Blur
  Sudden loss of sharpness across the whole frame usually means wind or accidental knocks.

📸 [Placeholder: Close-up showing good stars vs trailed stars]

Using Blink in PixInsight

1. Open the Blink tool.
2. Load your calibrated light frames.
3. Step through the subs one by one or run the sequence in slideshow mode.
4. Mark any bad frames for rejection.
5. Save the approved list to use in the Weighted Batch Pre‑Processing (WBPP) script or your chosen stacking software.

📸 [Placeholder: Screenshot of Blink interface with subframes loaded]

Best Practices

• Be Consistent: Apply the same standard to every frame so your stack has uniform quality.
• Err on the Side of Rejection: It’s better to stack fewer good subs than many mediocre ones.
• Don’t Panic About Minor Issues: Slight variations in sky brightness or faint trails can often be handled during integration. Focus on removing frames with significant defects.
• Keep Notes: If you notice recurring issues (e.g. focus drift or guiding problems), make a note to troubleshoot before your next session.

How Many Can You Afford to Reject?

There’s no fixed number – it depends on your target, total exposure time, and how noisy your data is. A rough guide:

• With a large dataset, be ruthless and reject aggressively.
• With only a handful of subs, you may need to accept minor imperfections.
• A single bad frame can cause more harm than the benefit of its signal.

Applying This Outside PixInsight

If you’re using other stacking software like Astro Pixel Processor or DeepSkyStacker, the principle is exactly the same. Always review your calibrated subs before integration, regardless of the tool.

Going Further: Subframe Selector

After Blink, many astrophotographers add a second, more technical screening step using PixInsight’s Subframe Selector. While Blink is fast and visual, Subframe Selector provides detailed measurements like FWHM (sharpness), eccentricity (star roundness) and signal-to-noise ratio. It can also weight your subs so the best frames contribute more to the final stack. This optional stage sits between Blink and WBPP and can make a significant difference for advanced projects.

Summary

• Blink is the quality control stage before stacking.
• Review every subframe for focus, trailing, transparency, and artefacts.
• Rejecting weak frames improves the signal-to-noise ratio and sharpness of the final master.
• Be consistent and don’t be afraid to discard poor data – fewer good subs beat many bad ones every time.

Next Step

With your best calibrated subs selected, the next article covers using Subframe Selector to analyse your frames in more detail and weight them for stacking.

📸 [Placeholder: Workflow diagram – Lights → Calibration → Blink → Subframe Selector → WBPP → Integration]