Understanding FITS, XISF & Linear Data

Introduction

When you open your stacked image in PixInsight, it may look unusually dark—and if you see a filename ending in .xisf instead of .fits, that’s completely normal.

This guide explains:

• What FITS and XISF formats are
• What linear vs non-linear data means
• How to safely view your data using ScreenTransferFunction (STF)
• Why this matters before further processing

FITS and XISF — Key Differences

• FITS (Flexible Image Transport System): Standard astronomy format — simple, reliable, widely compatible
• XISF (Extensible Image Serialization Format): PixInsight’s native format with metadata, STF storage, and compression support

📸 FITS vs XISF comparison screenshot

Linear vs Non-Linear Data

• Linear data: Raw, unstretched signal — appears dark and flat
• Non-linear data: Stretched for viewing and editing

Why It Matters Post-Alignment

Most PixInsight processes expect linear data:

• DBE, TGVDenoise, and colour calibration rely on it
• Stretching too early can destroy faint detail and colour accuracy

Viewing Without Altering Data

To inspect your image safely:

• Use ScreenTransferFunction (STF) for temporary visual stretch
• This does not modify your data
• Avoid permanent histogram stretching until later

📸 Linear vs STF preview example

When to Save vs Convert

• Use XISF: When working in PixInsight and saving progress
• Use FITS: For compatibility with other software

Summary

Next Steps

Next, you’ll learn how to evaluate your subframes using Blink, helping you remove poor frames before stacking.