What is the FTS file format?
.fts is one of three interchangeable extensions for the FITS (Flexible Image Transport System) format - the others being .fits and .fit. All three open identically; .fts appears most often on Windows systems where older software had trouble handling longer extension names.
FITS was developed in 1981 by Wells, Greisen, and Harten, formally endorsed by the IAU in 1982, and has been mandatory for NASA mission data archiving ever since. FITS Standard 4.0 (2016) is the current specification.
The format is self-describing: every file begins with the ASCII keyword SIMPLE at offset 0, confirming FITS conformance. Data is organized into sequential Header+Data Units (HDUs), each made up of 80-character ASCII header cards (36 cards per 2880-byte block) followed by a binary data array. The primary HDU carries the main dataset; extension HDUs can hold additional images, binary tables, or spectral data.
Key characteristics:
- Pixel depth is set by the
BITPIXkeyword:8,16,32,64(integer) or-32,-64(IEEE float) NAXISandNAXISnkeywords define dimensionality - from 1D spectra to 3D data cubes (wavelength × RA × Dec)- Optional tile compression via FPACK (Rice, GZIP, or HCOMPRESS algorithms)
- Optional
CHECKSUMandDATASUMkeywords provide Fletcher-32 integrity verification
FITS data cubes can reach several gigabytes. Programs such as DS9 (SAOImageDS9) and PixInsight are standard tools for viewing and processing .fts files in astronomical research.
Security & safety
RISK: LOWFITS is a scientific data format with no executable content. Safe to open. Malformed headers could theoretically crash vulnerable parsers, but no widespread exploits are known.
Format details
in a nutshell- Microsoft FrontPage Full-Text Search Index - FrontPage (discontinued 2003) used .fts for its web search index files - completely unrelated to astronomy FITS.
Programs that open FTS files
Windows4 apps
AvisFV (FITS Viewer by MSB Software)
Free File → Open; supports 8/16/32/64-bit integer and 32/64-bit floating-point FITS. Latest version 4.0 released March 2026.
DS9 (SAOImageDS9)
Free File → Open - full WCS, scaling, color maps, region overlays. Industry-standard astronomical viewer. 
macOS2 apps
Linux2 apps
FITSViewer.com
Free Upload FITS/FTS file; browser-based display with basic stretch controls. No install needed. Technical details
deep spec| MIME type | image/fits (also application/fits) |
| Magic bytes | ASCII "SIMPLE " (53 49 4D 50 4C 45 20 20) at offset 0 |
| Encoding | Binary data arrays + ASCII header cards (80 characters each) |
| Byte order | Big-endian |
| Block size | 2880 bytes per FITS block (36 header cards × 80 bytes) |
| Pixel depth (BITPIX) | 8, 16, 32, 64 (integer); −32, −64 (IEEE 32/64-bit float) |
| Pixel dimensions | Defined by NAXIS/NAXISn keywords; no hard limit |
| Supported data types | 1D spectra, 2D images, 3D data cubes (wavelength × RA × Dec), binary tables |
| Compression | Optional tile compression via FPACK - Rice, GZIP, or HCOMPRESS |
| Checksum | Optional CHECKSUM/DATASUM keywords using Fletcher-32 algorithm |
| Structure | Sequence of HDUs: primary HDU + optional extension HDUs for images, tables, spectra |
| Typical file size | 100 KB - 2 GB (single image to multi-extension data cubes) |
| Current standard | FITS 4.0 (2016); legacy files may conform to FITS 3.0 (1993) |
| Equivalent extensions | .fits (most common), .fit, .fts - all identical format |
| Released | 1981 (standardized by IAU 1982) |
| Latest version | FITS Standard 4.0 (2016) |
| Open standard | Yes · royalty-free |
| Specification | fits.gsfc.nasa.gov |
FTS conversions
Community Q&A
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