Image Inspectors

Siril has several tools that can help you to analyze your image and tell you about the quality of the shot. In particular if your setup has or not optical defects.


The first tool proposed by Siril is the tilt calculation. Sensor tilt occurs when the sensor is not orthogonal to the imaging plane: this requires an intervention on the optical system. You can execute this functionality in two different ways. Either via the GUI (in the menu Tools ‣ Image Analysis ‣ Show Tilt) or via the command line. The latter even offers an alternative that allows you to calculate the tilt over a whole sequence of images for greater accuracy. The following command:

Siril command line

tilt [clear]
Computes the sensor tilt as the FWHM difference between the best and worst corner truncated mean values. The clear option allows to clear the drawing

will output:

22:28:13: Running command: tilt
22:28:13: Findstar: processing for channel 0...
22:28:15: Stars: 7598, Truncated mean[FWHM]: 3.40, Sensor tilt[FWHM]: 0.31 (9%), Off-axis aberration[FWHM]: 0.39

In the console are indicated:

  • the number of stars used for the measurement

  • the average FWHM on the sensor, free of outliers

  • the tilt, expressed as the difference between the best and the worst FWHM on the 4 corners of the image with in parenthesis the percentage of tilt deviation (value greater than 10% indicates a tilt problem)

  • the aberration, expressed by the difference in FWHM between the stars in the center and the stars on the edges of the sensor

If the number of stars detected is low (<200), the dynamic PSF detection parameters allow improvement by adjusting the threshold / radius. In fact, the greater the number of stars used in the calculation, the more reliable the result of the analysis.


For the result to make sense, it is preferable to run this command on a sub and not a stacking result. A pre-processed image (just demosaiced for color sensors) is therefore ideal. Moreover, the drawn quadrilateral has its proportions exaggerated, in order to be more visible on the screen. It can’t correspond exactly to reality.


Display of the tilt diagram


As well as using the tilt -clear command, the tilt diagram can be cleared using the Remove button in the Dynamic PSF dialog.

Siril command line

seqtilt sequencename
Same command as TILT but for the sequence sequencename. It generally gives better results

Links: tilt

Aberration Inspector

This tool creates a 3x3 mosaic of the image center, corners and edges. This makes it easy to compare the shape of the star in different sections of the image. You can access this feature in the menu Tools ‣ Image Analysis ‣ Aberration Inspector. You can change the settings of this tool, to modify the size of the panels and the window, in the preferences.


Aberration inspector window showing aberration in the stars located in the edges because of the optical system.

It is also a very good indicator to know if the image contains a gradient: the differences in brightness becoming very visible.


Aberration inspector window showing differences in brightness.

Siril command line

Splits the loaded image in a nine-panel mosaic showing the image corners and the center for a closer inspection (GUI only)