With this tool you can check the focus or start an autofocus run. It can be used with motorized focuser or manual focuser.
If the autofocus mode is set to Planet all the reference to HFD are replaced by Sharpness but the tool work the same.
When focusing on a star, make first a preview exposure and double click on a non saturated star you want to use for focusing.
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 Sharpness or HFD / FWHM are at minimum value.
The star peak intensity should not be saturated. And the Signal/Noise ratio must be high enough.
When the autofocus is running it show a plot of the HFD or Sharpness 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 and Off-axis aberration (curvature) 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 HFD difference between the best and worst corner median values. In addition as a graphical indication it draws a 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 a few tenths looks normal but anything more indicates a camera mounting problem.
Off-axis aberration as the HFD difference 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. But be careful to first remove tilt and get a precise focus at the image center, otherwise this value is meaningless.