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Focus Tool

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.

Show star profile

When you double click on a star, it's profile and information about it's sharpness are show here.

The top part of the tool show three tabs with graphical representation.

The curve in the Profile tab is a cross-section of the star image intensity.
The narrower it is the better and sharper is the star image.
Take a specific attention to the top of the curve. A flat top indicate a saturation, a rounded top is indication of bad focus or oversampling.
A curve that enlarge too much on the foot can be an indication of optical issue, light cloud in the sky, or frost on the lens.

The 2D tab is an extreme zoom on the star image colorized to enhance the view.
The area in green has a level higher than half maximum, it represent a 2D view of the FWHM.
The area in yellow has a level higher than 9% above the background.
The area in red is the foot of the profile curve with a level between 3% and 9% above the background.
Look for the roundness of the image with each color area as narrow as possible.

The Trend tab show the history of the previous measurement.
This is the main help for the manual focus tool, adjust the focuser position to reach the minima of the red curve and the maxima of the green. The HFD is in red and the intensity in green.
Double click on the graphic to clear and start a new graph.
Move the mouse cursor on the curve to show the exact value in the status bar.

Each of this graphic can be opened in a separate window with a click on the button on the bottom right. This allow make the graph larger and to monitor both at the same time, even when the Focus tool is not show.

Measurement values

Below are the measurement value for the current star image.

The HFD (Half Flux Diameter) unit is pixels. The history of the previous three measurement is show on the right.

The FWHM (Full Width at Half Maximum) units are pixels and arc-seconds.

The star peak intensity should not be saturated. And the Signal/Noise ratio must be high enough.

Manual focus

When focusing on a star, make first a preview exposure and double click on a non saturated star you want to use for focusing.

Click Manual focus aid to start a preview loop on the selected star area, configurable by setting the focus window size.

Optimum focus is reached when the Sharpness or HFD / FWHM are at minimum value, and the intensity is at the maximum value.


Click Autofocus to start the auto-focus procedure on the selected star. You must have configured the auto-focus in advance and if applicable run the V curve learning tool and saved the result.
This button do not execute the action that are specific to autofocusing from a sequence, it will not stop autoguiding, and not slew to a nearby bright focus star.

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.

Vcurve Dynamic / Planet Iterative

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.

You can review this graph later using the button View last autofocus.

Image inspection

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.

en/documentation/star_profile.txt · Last modified: 2021/05/01 17:58 by pch