Handling FITS/XISF and Understanding Linear vs Non‑Linear Data

Introduction

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 normal. This guide focuses purely on explaining:

• What FITS and XISF formats are and when to use each
• What linear versus non‑linear data means
• How to view your data safely using ScreenTransferFunction (STF)
• Why understanding these distinctions is crucial before moving forward

FITS and XISF — Key Differences

• FITS (Flexible Image Transport System) is the standard format in astronomy—simple, reliable, and widely compatible. It doesn’t support PixInsight-specific metadata or compression.
• XISF (Extensible Image Serialization Format) is PixInsight’s native format. It supports metadata like processing history and embedded STF, offers optional lossless compression (e.g. LZ4), and generally produces smaller file sizes.

📸 [Placeholder: Screenshot showing a FITS file and an XISF file opened in PixInsight]

Linear vs Non‑Linear Data — What’s the Difference?

• Linear data is the raw, unstretched signal from your calibrated and stacked subs. It appears dark and flat on-screen, as no contrast or stretch has been applied.
• Non‑linear data is what transformed, stretched images look like—ready for colour adjustment, editing, and sharing.

Why It Matters Post-Alignment

Most PixInsight processes assume your image remains in a linear state:

• Tools like DBE, TGVDenoise, or Photometric Colour Calibration rely on linear data to work reliably.
• Converting to non-linear too early—via histogram transformations or similar—can compromise faint details and colour integrity.

Viewing Without Altering

To inspect a linear image without changing it:

• Use the ScreenTransferFunction (STF). It provides a temporary visual stretch without altering data. Even if you save as XISF, the image remains linear.
• Avoid histogram transformations until after you’ve completed calibration, stacking and noise control.

📸 [Placeholder: Screenshot showing linear view vs STF preview in PixInsight]

When Should You Save vs Convert

• If you’re working across multiple sessions or planning long-term edits, save in XISF to retain metadata and STF history.
• If you export to other software or need compatibility, FITS remains entirely valid and works perfectly in PixInsight workflows.

Summary

Next Steps

Once you’re familiar with file formats and data states, the next article will show you how to visually assess your subframes using Blink, helping you to cull out poor frames before stacking for the best possible results.