In 2022, Han Kleijn, developer of the ASTAP software, offered to contribute to the development of a new pseudo-standard using the TIFF format and taking advantage of the power of FITS file headers. This is how the Astro-TIFF format was born.

Why a new standard among all the others?

Currently, the most used format for astrophotography is the FITS format. This one, developed by processional scientists, meets all the expectations of amateurs. And although its great flexibility causes some concerns of compatibility it remains the format to be preferred.

Other specialized formats exist but are usually associated with a specific software. Like the XISF format developed by the PixInsight team. This last format, although often requested in Siril, is a format dedicated to PixInsight, a proprietary software. So the interest in developing compatibility with Siril is minimal, and we've only done it for reading in the 1.4.x cyle.

Developing Astro-TIFF appears then as a good alternative, because based on the TIFF format, it is possible to open the files on any image processing software.

Finally, the TIFF format supports compression (as does the FITS format) which allows for smaller image sizes.

Specification 1.0

Dated 2022-06-21

  • Files are following the TIFF 6.0 specification including supplement 2 fully.

  • The FITS header is written to the TIFF baseline tag Image Description. Code 270, Hex 010E.

  • The header is following the FITS specification except that that the lines can be shorter then 80 characters and lines are ending with either CR+LF (0D0A) or LF (0A).

  • First line in the description is the first header line and starts with SIMPLE. The last line of the header starts with END.


  • TIFFtag_orientation=1 (left-top) Orientation is following the conventions. Pixel FITS_image[1,1] is left-bottom. TIFF_image[0,0] is left-top. These pixels are first written or read from the file. So when writing a FITS image into TIFF preserving the orientation for the user, the first pixel to write is FITS_image[1,NAXIS2].

  • TIFFtag_compression=8 (Deflate) or 5 (LZW).

  • For greyscale images TIFFtag_PhotometricInterpretation = 1 (minimum value is black, maximum is white).

  • Write all available header keywords.


  • This use of TIFF format is intended for 16-bit lights, darks, flats and flat-darks (astronomical images), but can also be used in the 32-bit format. It is possible to convert FITS to TIFF and backwards but the application programmer can decide to export only (write) or only import (read) in Astro-TIFF format.

  • If an astrometrical (plate) solution is included then it should match with the image orientation.

  • Some header keywords are redundant like NAXIS1, NAXIS2, BZERO and BITPIX and are not required. TIFF image dimensions and type are leading.

  • The de-mosaic pattern specified in the header should match with the image orientation.

  • The header will be visible in many image manipulation programs.

Example of an Astro-TIFF header that looks just like a FITS file header:

SIMPLE  =                    T / file does conform to FITS standard
BITPIX  =                  -32 / number of bits per data pixel
NAXIS   =                    2 / number of data axes
NAXIS1  =                 6248 / length of data axis 1
NAXIS2  =                 4176 / length of data axis 2
NAXIS3  =                    1 / length of data axis 3
EXTEND  =                    T / FITS dataset may contain extensions
COMMENT   FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT   and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H
BZERO   =                    0 / offset data range to that of unsigned short
BSCALE  =                    1 / default scaling factor
DATE    = '2022-12-14T16:05:47' / UTC date that FITS file was created
DATE-OBS= '2022-05-06T20:29:20.019000' / YYYY-MM-DDThh:mm:ss observation start,
INSTRUME= 'ZWO CCD ASI2600MM Pro' / instrument name
OBSERVER= 'Unknown '           / observer name
TELESCOP= 'iOptron ZEQ25'      / telescope used to acquire this image
ROWORDER= 'TOP-DOWN'           / Order of the rows in image array
XPIXSZ  =                 3.76 / X pixel size microns
YPIXSZ  =                 3.76 / Y pixel size microns
XBINNING=                    1 / Camera binning mode
YBINNING=                    1 / Camera binning mode
FOCALLEN=              370.092 / Camera focal length
CCD-TEMP=                 -9.8 / CCD temp in C
EXPTIME =                  120 / Exposure time [s]
STACKCNT=                  126 / Stack frames
LIVETIME=                15120 / Exposure time after deadtime correction
FILTER  = 'Lum     '           / Active filter name
IMAGETYP= 'Light Frame'        / Type of image
OBJECT  = 'Unknown '           / Name of the object of interest
GAIN    =                  100 / Camera gain
OFFSET  =                   50 / Camera offset
CTYPE1  = 'RA---TAN'           / Coordinate type for the first axis
CTYPE2  = 'DEC--TAN'           / Coordinate type for the second axis
CUNIT1  = 'deg     '           / Unit of coordinates
CUNIT2  = 'deg     '           / Unit of coordinates
EQUINOX =                 2000 / Equatorial equinox
OBJCTRA = '09 39 54.932'       / Image center Right Ascension (hms)
OBJCTDEC= '+70 00 10.118'      / Image center Declination (dms)
RA      =              144.979 / Image center Right Ascension (deg)
DEC     =              70.0028 / Image center Declination (deg)
CRPIX1  =               3123.5 / Axis1 reference pixel
CRPIX2  =               2088.5 / Axis2 reference pixel
CRVAL1  =              144.979 / Axis1 reference value (deg)
CRVAL2  =              70.0028 / Axis2 reference value (deg)
CD1_1   =         -0.000580606 / Scale matrix (1, 1)
CD1_2   =         -4.12215e-05 / Scale matrix (1, 2)
CD2_1   =         -4.11673e-05 / Scale matrix (2, 1)
CD2_2   =          0.000580681 / Scale matrix (2, 2)
PLTSOLVD=                    T / Siril internal solve
HISTORY Background extraction (Correction: Subtraction)
HISTORY Plate Solve

Saving Astro-TIFF in Siril

In Siril you can save Astro-TIFF files by choosing the TIFF format in the save dialog when you click on Save As. The drop-down list in the TIFF dialog allows you to choose between saving in standard TIFF format or in Astro-TIFF format. The latter is the default format.

Astro-TIFF dialog

Save Dialog with Astro-TIFF option

Siril command line

savetif filename [-astro] [-deflate]
Saves current image under the form of a uncompressed TIFF file with 16-bit per channel: filename.tif. The option -astro allows saving in Astro-TIFF format, while -deflate enables compression.

See also SAVETIF32 and SAVETIF8

Sample files