CCDciel is free software released under the terms of the
GNU General Public License
This function help you to set the auto-focus parameters specific to your focuser, telescope and camera because it is not immediately evident what to set for all this numbers.
The principle is you start by centering and manually focusing a star that is used by the procedure to find how the star diameter change with an increasing movement of the focuser.
At the end it propose you new auto-focus parameters you can save to the configuration.
If you use an absolute position focuser it set to use the V curve focusing method and give you the opportunity to learn a new curve using the same star you already centered in the image.
To start the procedure select the menu File → Focuser calibration
The first screen give you information about centering, focusing and correctly exposing the star.
Do it now, before to click the Next button.
The next screen let you select the direction you want to move the focuser in the auto-focus operations. Depending on you setting the focuser can work better when moved in or out of focus. If you want to configure the temperature compensation, set the direction to compensate when the focuser cool down during the night.
If the focuser need a backlash correction you can enter the value now. But be sure to let it disabled if the compensation is done elsewhere.
Indicate the number of additional steps to use for the compensation, this must be greater than the actual backlash, don't hesitate to use a large value.
Then it offer you to change two measurement parameters, the maximum defocused star diameter (HFD) it will try to reach, and the initial movement of the focuser in step.
You can change the first parameter if you know what your equipment can achieve, otherwise keep the default and follow the recommendation if the first try fail.
The minimum movement is not really important to change because any way the program will double the value until it see enough change in the star diameter. You can just save a few time if you know your focuser need more steps even for fine focus.
It start slowly, just wait until it reach the maximum diameter.
Then it start to move the focuser on the opposite side, much quickly as it know what speed to use.
You can check if the curve is symmetrical and as not evidence of gross error.
Now the program know how your focusing system react.
You can review the parameters before to click Next to save them to the configuration.
Later you can view or change this values in the auto-focus parameters.
The procedure is completed and you can close this window.
With an absolute position focuser the focusing method is set to Vcurve and now you must learn a new curve for the new parameters.
The Vcurve learning automatically open, already filled with the correct setting. Do not change any value and just click the Learn button and after it complete click the Save button.
The initial setting is now completed.
You can now open the auto-focus parameters and without changing any of the detailed parameters you can change the auto-focus method between Vcurve and Dynamic depending on your preferences.
Select also how you prefer to run the auto-focus automatically as part of a sequence.
You have the choice to stay at the current target position and use whatever stars are present, or slew to a nearby star of fixed magnitude.
Later you can also alter this choice target by target in the sequence editor.
If you use the Vcurve method it is strongly recommended to slew to a focus star.