Practical Field Guide: How to Inspect & Edit Astronomical FITS Metadata
A hands-on troubleshooting guide for resolving stacking failures, debayer errors, plate-solving issues, and mislabeled filter tags in astronomical FITS files.
β οΈ Common Real-World Scenarios Requiring Header Editing
Whether you capture data with N.I.N.A., ASIAIR, KStars/Ekos, or proprietary observatory software, metadata errors frequently corrupt post-processing pipelines. Below are the most frequent issues experienced astronomers encounter:
β Missing or Corrupt BAYERPAT
Color cameras (OSC) require a valid BAYERPAT (e.g. RGGB, BGGR). If missing, Siril or PixInsight produces monochromatic or inverted color images during debayering.
BAYERPAT keyword to match your sensor's color filter array pattern.β Plate-Solving Failures (WCS / Scale)
Plate solvers (ASTAP, ASTROMETRY.NET) fail when telescope focal length (FOCALLEN) or sensor pixel size (PIXSIZE / XPIXSZ) are zero or incorrectly recorded.
β Unassigned Filter Metadata in LRGB / SHO
Mono imaging sequences occasionally lose the FILTER tag, causing automated stacking tools to blend Narrowband Ha with Luminance or Red channels.
Ha 3nm, OIII, Lum) on all relevant subframes.β Inconsistent Exposure Time (EXPTIME)
When stacking with sigma-clipping or exposure weighting, zero or mismatched EXPTIME values distort signal-to-noise ratio (SNR) algorithms.
π οΈ Visual Step-by-Step Walkthrough
Here is how you can inspect and modify these parameters safely using the zero-friction online editor:
Step 1: Open and Inspect the File
Load your subframe or master light into the browser. The file is parsed completely client-side:
Step 2: Locate, Modify, or Add Missing Keywords
Use the structured table to make instant changes without writing scripts or opening terminal windows:
Pro-Tip for Fast Editing: Click the Quick Helpers buttons at the top of the table (such as + OBJECT, + EXPTIME, + FILTER, + BAYERPAT, + GAIN). The editor automatically fills the standardized keyword name and opens the input field with the proper data type pre-selected.
π Standard Astronomical FITS Keywords Reference Table
Refer to this reference when modifying standard header cards to maintain compatibility with NASA and IAU guidelines:
| FITS Keyword | Data Type | Example Standard Value | Scientific Description & Function |
|---|---|---|---|
SIMPLE |
Boolean | T |
Indicates whether the file conforms strictly to the standard FITS specification. Must be first card. |
BITPIX |
Integer | 16 / -32 |
Bits per data pixel (16 = unsigned/signed short; -32 = single-precision IEEE float). |
NAXIS |
Integer | 2 (or 3) |
Number of coordinate axes (2 for 2D astronomical image; 3 for datacubes/RGB). |
OBJECT |
String | 'M42 Orion Nebula' |
Target celestial object name for cataloging and project identification. |
EXPTIME |
Float / Number | 300.0 |
Total exposure duration of the observation measured in seconds. |
FILTER |
String | 'Ha', 'OIII', 'Luminance' |
Optical filter utilized during the capture sequence. |
BAYERPAT |
String | 'RGGB' / 'BGGR' |
CFA (Color Filter Array) pattern required for proper debayering. |
FOCALLEN |
Float / Number | 800.0 |
Effective focal length of the telescope/lens optical train in millimeters. |
PIXSIZE |
Float / Number | 3.76 |
Physical pixel pitch of the sensor active area in microns (ΞΌm). |
GAIN |
Integer / Float | 100 |
Sensor amplifier gain setting utilized during frame acquisition. |
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Open FITS Header Editor Online βπ Related Resources
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