CCDciel is free software released under the terms of the
GNU General Public License
With this tool you can check the focus or start an autofocus run. It can be used with motorized focuser or manual focuser.
Make first a preview exposure and double click on a non saturated star.
The top curve is a cross-section of the star image. Try to make it as narrow as possible.
Below the previous relative HFD measurement in red and the intensity in green.
The HFD (Half Flux Diameter) unit is pixels.
The FWHM (Full Width at Half Maximum) units are pixels and arc-seconds.
Optimum focus is reached when HFD and FWHM are at minimum.
The star peak intensity should be not be saturated.
When the autofocus is running it show a plot of the HFD versus the focuser position. The aspect of the plot depend on the selected focusing method.
If you move the mouse over the graph it display the values of the focuser position and HFD.
You can click a button to not close the graph when the auto-focus is completed to let you more time for examination. Click the button again to close the graph.
Click Image inspection to detect the stars in your image and indicate the HFD for each and plot a tilt indicator in the image.
The tilt is indicated in the log as % ratio between the best and worst corner median values. In addition as an graphical indication it draws an trapezium in the image based on the four median values.
There can be some variation in images of the same series, so a tilt of maybe 20% looks normal but anything more indicates a camera mounting problem.
The message in the log also display a curvature indicator that show the ratio between the center and the corner of the image. This is an help to adjust the optimal distance between the field flattener and the camera.