Three sequence file types
A set two or more FITS files⚓
Siril uses natively 16-bit unsigned integer data or 32-bit floating point data for the FITS images, other formats are automatically converted.
Note
The file extension for FITS files can be configured in the settings, by default it is .fit. Changing it will change the extension used to detect FITS sequences in the current working directory, but also the extension of the files created by Siril in conversion or any processing that creates an image.
A single SER file⚓
SER is a format meant to contain an acquisition sequence of several contiguous images in a single file. It is a rather simple format that cannot contain as much metadata as FITS, but more than simple films and data is not compressed. SER files can contain images of 8 or 16 bits per channel only. There are three types of SER files, depending on the pixel content: monochrome, CFA or color (3 channels).
See https://free-astro.org/index.php?title=SER for more information on the SER format and why film formats like uncompressed AVI should not be used for astronomy.
WarningTo some extent, a regular film file such as AVI or any other container are supported too⚓
Film files support is being dropped in favour of SER, but it can still be useful to open a film in Siril, to explore its content, extract some frames or convert them.
A few operations can still be done, but in a slower way than with other sequences, like sum stacking. For a complete processing you will face limitations and incompatibilities.
A single FITS file⚓
Also called FITS cubes or FITS sequences or FITSEQ for short in siril.
The FITS format is an image and science data container, it can contain several of these in a single file. We can use that to store an entire sequence for FITS images in a single fils without losing the FITS header information for each image. It is a file format that professional astronomers use.
It's simpler to manage one file on the disk than 2000, but since it is a single file, special care had to be taken to process the images in parallel, and the result is that it is slower to user FITSEQ than a sequence of FITS files.
It is also an alternative to SER for a single-file sequence, with 32 bits per channel and full header support.
Tip
The file extension for FITS files can be configured in the settings, by default it is .fit. Changing it will change the extension used to detect FITS sequences in the current working directory, but also the extension of the files created by Siril in conversion or any processing that creates an image.
Note
You can force Siril to recompute all sequences in the working folder, by checking the Force .seq recomputation box. Be careful, however, that this erases all the data (normalization, alignment, ...) which have been calculated before.