Sequences are what Siril uses to represent a set of manipulated files, for example the set of dark images that we'll turn into the master dark. It is a very useful tool for handling a large number of files that need to be linked with each other.
A set of two or more FITS files
Siril uses natively 32-bit floating point data or 16-bit unsigned integer data for the FITS images, other formats are automatically converted. To be recognized and detected as a sequence, FITS images file names must respect a particular pattern which is:
basename can be anything using ascii characteres. It is usually convenient, but not mandatory, to have it end with the
_character. It will be used as the sequence name.
$i is the index of the image. It must be a positive number and can have several leading zeros.
[ext] is the supported extension as explained in the settings,
The extension used to detect FITS sequences in the current working directory will be the same as the extension configured in the settings and as the files created by Siril.
Some operating systems limit the number of images that can be opened at the same time, which is required for median or mean stacking methods. For Windows, the limit is 2048 images. If you have a lot of images, you should use another type of sequence, described below.
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).
An SER file can be opened either via File and Open, or with the Search sequences button.
See here <file-formats/SER:SER> for more information on the SER format and why film formats like uncompressed AVI should not be used for astronomy.
To 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 of FITS images in a single file while preserving the FITS header of each image. It is the 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, some operations on single images of the sequence may not be possible. In particular, it is not currently supported within Siril to change the header of a single image.
This format is an alternative to SER for a single-file sequence, with 32 bits per channel and full header support.
Loading a sequence
When the working directory is set in the right place, the FITS follow the correct nomenclature, and the extension of the FITS files is also set correctly, then click the Search Sequence button of the Sequence tab. A drop-down list opens with all the sequences available in the folder. If only one is found, then it is automatically selected and loaded.
A great strength of Siril is that it easily manipulates image sequences. When a sequence is opened, the reference image (see below) will be displayed, by default this is the first image. However, it can sometimes be useful to inspect individual images of a sequence. This is possible with the frame selector, available via the toolbar with the button or via the sequence tab with the Open Frame List button.
Clicking on an image in the list will load it and display it in the main area, while keeping the sequence as the active object for processing. More than just a image display selector, the tool can also be used to manually exclude images from the sequence, or visualize which are still included, visualize the values of image quality and shift between images if they have been computed, and change the reference image. Note that more image quality information can be viewed in the Plot tab.
Excluding an image from the sequence does not mean its data will be permanently deleted, it will just not be used for the subsequent processing operations, if instructed to do so. In most cases, the option to look for is called Process included images only.
The reference image is the image in the sequence that will serve as target for the registration and for the normalization. Other images will be transformed to look like the reference image, so it should be chosen carefully. Fortunately, since Siril 1.2, a new two-pass registration can automatically select the best image of the sequence as reference image before proceeding to image transformation.
The header bar of the window will provide many controls for these sequence properties:
The drop-down menu allows the channel for which registration data (quality, shifts) is displayed to be changed, if they exist for other channels.
The first button of the toolbar sets all images of the sequence as manually excluded.
The second one, sets them all as included.
Siril command line
select sequencename from to
Siril command line
unselect sequencename from to
The third includes or excludes the images selected from the list (multiple selections can be done with Ctrl or Shift) of the sequence.
The last button can be deactivated to not show the red rectangle over registered images. It represents the framing of the reference image as computed by the registration.
The button Reference image is used to select the reference image for the sequence. All sequences must have one, it will be the first image if unset or by default.
Siril command line
setref sequencename image_number
Finally, the search field allows you to find the images by name.
It is also possible to sort all the images by clicking on the column headers. Thus you can sort the images by their name, their number, their X/Y offset or their FWHM. The latter is very useful to have a look at the best and worst images.
The Sequence Export tool allows you to export a sequence of images in a
variety of formats. It is particularly useful if you want to export the images
taking into account the registration information contained in the
with optional cropping and normalization.
With the sequence export function, you can select a sequence to export, choose the file format and compression level for video formats. Siril's sequence export function supports a wide range of image file formats, including FITS (single FITS file or sequence FITS file), TIFF, SER , AVI, MP4 and WEBM and can come in handy when building timelapse.
The button Normalize images allows you to normalize the images with respect to the reference image. The normalization is the same as the one done during the stacking, with the following settings: Additive with scaling, Faster normalization disabled.
Moreover, it is possible to play with the image filtering criteria to exclude or not images according to their quality. A button Go to the stacking tab has been added here, to easily go to the tab that exposes them.
All sequence information, registration transformation, statistics and frame
selection are stored in a
.seq file saved next to the sequence files. It is
strongly recommended never to edit this file manually because Siril writes
continuously inside it and one wrong character could make the reading of the
One way to clean the content of this sequence file is to go in the Sequence tab and click on Clean Sequence. The choice of what will be cleaned can be defined by clicking on the small arrow next to it.
Siril command line
seqclean sequencename [-reg] [-stat] [-sel]