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Focuser Tool

With this tool you can control your motorized focuser manually. The control are adapted if you use an absolute or relative focuser. The temperature is reported if available.

With a relative focuser you can set the desired movement in steps, then click the arrow buttons < > to move the focuser in or out.

With an absolute position focuser you can set the movement as with the relative focuser, or set an absolute position and click Move to.

The button V-learn start the V-curve learning.

V curve learning

If you not already do it, it is strongly suggested you use the focuser calibration wizard to set the correct default parameters before to use this procedure.

Introduction
In case you have an absolute position focuser and want to use the V-curve auto-focus you have to run the V-curve learning routine first.

The routine steps the focuser and measures at several positions the size (HFD value) of a star. This will result in a typical V-curve of two lines left and right of the focus point, The slope of the lines will be used later in the autofocus routine for finding the focus. in this screenshot the routine will step from position 12186+800 to position 12186-800.

The focus position shown as 12186 should be approximately the best focus position. The maximum offset 800 as shown should be selected such that the HFD value at the beginning and end is above the Start HFD focus=20 so in this example 25.

Steps to follow
If you are using filters, select the luminance filter. Start an preview loop of about one second exposure and center on and select a bright star. Run the manual focus aid and check if the peak intensity indication is about 80% of saturation level using the star_profile tool. If not adapt the exposure accordingly or select an other star.

Be sure to stop the preview loop and press the V-learn button

The first time you launch this tool all the settings and graphic are empty.

Indicate the most accurate focus position you can estimate, as you make a manual focus just before you can click the Get current button here.

Set the Max offset value such that moving to the best focus position plus this offset give a defocused image with an HFD around 25.

Set the number of step to 30, this is a good value that allow for precision curve. But for the first try you can save some time by setting it to 10. Just remember to repeat with 30 when you are accustomed with the procedure.

When this three numbers are set click the Learn button on the bottom.

This curve is slowly build up until the procedure is finished. Progress in is also reported in the Log and Star profile tools.

After the measurement are completed the curve should look as in the screen shot here. The graph shows the HFD value as function of the focuser position.

Check the following important points before you continue:

  • The curve must be centered with top left and right about at the same level focus in the middle.
  • It must extent to about the double of the value of Near focus HFD in the options, indicated by a blue line here.
  • The two branches of the V must show a long linear part, specifically on the side of the base of the focus direction arrow (right on the screen shot).
  • The branches must not flatten on the top. If you remark such behavior you must reduce the Max offset value.
  • The move direction should be such that the learn routine moves the focuser in the same the direction as you have to adjust the focuser manually during the night when it is cooling down. If not change the direction in settings. For a reflector (mirrors) type of telescope you typically have to turn the focuser out if the temperature drops. For a refractor (lenses) type of telescope you typically have to turn the focuser in.

At the center of the V with the points marked in red.
It is important to exclude this points from the linear regression and you can use the Fit cursor to exclude more or less points.

When you adjust this cursor, look to maximize the Quality value on the right but also check the aspect of the black regression line across the green circles. The linearity must be specially good at the intersection with the blue line.

When you are satisfied by the result click the Save button. This will save the curve parameters and mean temperature for use in the auto-focus routine.

You can now try the Autofocus in the Star profile tool.

en/documentation/focuser.txt · Last modified: 2018/03/17 16:18 by pch