Intensity Profiling
Siril has an intensity profiling mode. The user selects a line between two points and Siril will generate a graph of the pixel values between them. This has several uses. It can be used to inspect the intensity profile of an individual star or a whole galaxy.
Basic Intensity Profile
To make a basic intensity profile of a star or other object, select the Profile button in the bottom toolbar. This puts Siril into profiling mode and opens a small dialog.
You can now click and drag on the main image display to set the start and finish points of the line you wish to profile. If you hold down the Shift key while dragging the line, it will snap to be either horizontal or vertical.
Tip
When the profile line is exactly horizontal or exactly vertical, exact pixel values can be used directly from the image. When the profile line is neither horizontal nor vertical, the points to be plotted do not fall exactly on a pixel and bilinearly interpolated pixel values are therefore used.
A custom title for your plot can be entered in the control at the bottom of the dialog.
Tip
When processing a sequence, it is possible to have the custom title display the image number and total by adding () to the end of the title. For example entering Solar Spectra () as the title for a 5 image sequence will generate titles Solar Spectra (1 / 5), Solar Spectra (2 / 5) etc. The brackets are ignored and removed if processing a single image.
Types of Profile
Use the radio buttons to select the type of profile you want. (Click on the example images below to see them full size.)
Mono profile. For mono or color images, generate a luminance profile between two points. This mode can be used with spectrometric data.
Tip
If a color image is loaded but the mono profiling mode is selected, the profile will be made according to the viewport. The R, G and B viewports provide mono profiles of their respective channel and the RGB viewport provides a luminance profile weighting all 3 channels equally.
Color profile. For color images, generate three profiles for the R, G and B pixel values between two points. This mode can be used with spectrometric data.
Tri-profile (mono). For mono or color images, generate three parallel equispaced luminance profiles between two points. The spacing between the 3 profiles can be set using the spin button.
CFA. For images with a Bayer pattern only, generate four profiles for the four CFA subchannels between two points. This can be particuarly useful for inspecting the profile of Bayer patterned flats or other Bayer pattern images before they are debayered.
This image demonstrates use of the Custom Title control to set a custom title for the plot.
Click Apply to generate your profile.
Precise Coordinate Entry
In order to make it easy to input coordinates precisely and repeatably, a manual entry method is provided. Click the Manual Coordinates button and you can enter the X and Y coordinates of the start and end points of the profile line. If a profile line is already drawn but one point is not quite in the place you want it, you can use this popup dialog to fine tune the placement of the endpoints.
If you wish to set an endpoint exactly to the position of a star, make a rectangular selection around the star and click the relevant star button to the right of the dialog.
Note
When using the CFA mode, coordinates are given in the input image. However each CFA channel is half the width and half the height. The x axis in the CFA mode plot is measured in pixels in the CFA subchannel, i.e. it will span half the number of pixels that it does in the input image.
Measurement
The intensity profile line can be used as a measuring tool in two ways:
Checking the Measure profile checkbox will measure all profile lines dragged with the mouse, similarly to the Ctrl + Shift + Drag quick measurement function.
In the Coordinates dialog there is a Measure button. This provides the same measurement function but allows you to set the endpoints exactly, and then measure the profile line on demand. By selecting stars, minor planets or comet nuclei as end points as described above, measurements between two celestial bodies can be made very precisely (with sub-pixel precision).
Here, two close stars have been selected and set as the endpoints and the separation between them measured as 5.2 arcsec. This could be used to study close binaries or to triangulate the position of a minor planet.
Note
Siril's measurement function makes the small angle approximation for the angular separation \(\theta\). The most significant error term is proportional to \(\theta^3\) and is less than 1% for measurements up to 10°: it is therefore valid for most astrometric uses, but will become inaccurate for large measurements across ultra-wide field images. A warning will be written to the log for measurements over 10°.
Siril Plot Tool
The profiling feature uses Siril internal plotting tool to display the different
profiles. With the *.dat files produced, you can still use any plotting
tool of your liking to explore the underlying data.
A right-click anywhere in the plotting surface will pop-up a contextual menu to:
Show/hide grids and legend
Export current view to clipboard,
*.pngor*.svgSave underlying data to a
*.datfile
Siril plot contextual menu
Note that all exports account for the current zoom/pan while saving to dat will export unfiltered data.
The following GUI interactions are avalaible:
Click + Drag to draw a selection. The zoom is set to the selected zone when the mouse is released.
Ctrl + Drag to pan the current view.
Ctrl + Scroll to zoom in/out.
Double-click to reset to the default position/zoom.
Commands
Siril command line
profile -from=x,y -to=x,y [-tri] [-cfa] [-arcsec] { [-savedat] | [-filename=] } [-layer=] [-width=] [-spacing=] ["-title=My Plot"]
Siril command line
seqprofile sequence -from=x,y -to=x,y [-tri] [-cfa] [-arcsec] [-savedat] [-layer=] [-width=] [-spacing=] [ {-xaxis=wavelength | -xaxis=wavenumber } ] [{-wavenumber1= | -wavelength1=} -wn1at=x,y {-wavenumber2= | -wavelength2=} -wn2at=x,y] ["-title=My Plot"]