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Supported FITS files by Siril
In versions 0.9.x, the native image file format of Siril was unsigned 16-bit FITS (USHORT), one or three layers only, with data stored bottom-up.
Since the introduction of the 0.99.x series, a second native image file format has been introduced, the 32-bit floating point FITS (FLOAT), normalized to 1, so with values between 0 and 1, one or three layers only and with data stored bottom-up as well. It was a widely requested feature, and its main purposes are to increase the precision that results from image processing and to support negative values on bias/dark subtraction.
This new 32 bits format became the default format since its introduction, but since it doubles the requirements for available memory and disk space and slows the processing down, it can be disabled in the preferences. In that case, the behaviour is the same as in older 0.9.x versions.
The 32 bits format is not used in conversion when not needed. In general, acquired images are 8 or 16 bits deep, so for these, Siril still uses the 16 bits format. For preprocessing, the images are then converted to 32 bits.
Siril can also use FITS compression. It is disabled by default, see the preferences to enable it. It saves disk space, but requires more processing power to compress images when writing them and to decompress them when reading them back.
Bitpix: pixel precision
Siril can work with other FITS formats, meaning that it can load, display and process them just like the file formats above, but they will be converted to either formats on loading:
- 8-bit integers per channel: the images are converted to 16 bits (USHORT) in memory, no problem
- 16-bit integers per channel: the signed SHORT images are converted to unsigned short, and no problem either for 15 bits formats. USHORT is in fact stored as SHORT with a 2^15 offset.
- 32-bit integers per channel: the image is converted to the floating point 32-bit format. Data is normalised with the maximum value, and the minimum value is used as an offset. This is fine to use a single image, but not for a sequence, as min and max value change among frames, the consistency of the data is not preserved.
- 64-bit integers per channel: not currently supported, it would almost be sure to give unusable data.
- 32-bit and 64-bit floating point numbers: the image is loaded as Siril's 32-bit image format, but if the input data is not normalized to 1, the data is divided by 65535.
FITS cube
Since the 0.99.x versions, Siril provides another way to save a sequence of images with the FITS format: the FITS cube, or fitseq as we call it sometimes; it is a single FITS file containing several FITS images. Siril only uses these files to store its own sequences, meaning that all images inside the file must have the same type of data, or the same dimensions and pixel format. The same FITS formats are supported as for regular FITS images.
Professional observatories provide data in form of a FITS cube, with a lot of metadata and various types of data. Often, the image data is not registered as the primary content of the file. Siril may be able to open the file as a sequence, and may be able to display an image from it. Using a sequence of these files will not work, it may only be possible to display or extract an image from such a file.