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. And the Signal/Noise ratio must be high enough.
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. For dynamic focus the pink curve is the resulting hyperbola fit.
If you move the mouse over the graph it display the values of the focuser position and HFD.
You can click the button to open the graph in a separate window and 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 measure the median hfd value, sensor tilt and curvature.
The routine will detect and annotate the stars with their HFD value and plot a tilt indicator in the image.
In the log the following will be reported:
Image median hfd which is an excellent indicator of the quality of focusing. The lower the value the better the focusing, the sharper the stars are. The value is also depending on the astronomical seeing and the quality of the optics.
Sensor tilt as the % 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.
Curvature as the % ratio between the center and the corners of the image. This is an help to adjust the optimal distance between the field flattener and the camera.