MLUnsold Digital Imaging

Auto focus training is used to parameterize the focus properties of your telescope, camera, focuser, and ASCOM absolute position focus motor. Training parameter setup is simple if the following guide is followed.

It is best to create training parameters for each bin size used with a given telescope, camera, focuser, and ASCOM focus motor.
Use autoguiding when training so that the test star is not lost.
Use an isolated star that is not overexposed or saturated. A good star for training and auto focus use has a minimum or in focus half flux diameter ( HFD ) between ~ 1.7 and 4.0.
Brighter stars can be used for training and auto focus but you may need to adjust the exposure to force the star's HFD to be in the range ~ 1.7 to 4.0.
A star with HFD 2.0 with 1 second exposure is ideal.
Use the Focus tab of IP camera control and experiment with your scope and focuser to determine how many focus motor steps are required to change the HFD of a star by ~ 0.5 to 1.5 HFD. The number of steps required will vary greatly depending on the focus motor and telescope. With a SBIG ST2000XM, RoboFocus motor, and 130mm F6 TMB the step size is only 4 but the step size could be 150 for some setups.
Use the step size to calculate how many steps are required to cover a HFD test range from an in focus star to a star with HFD no larger than 15.0. The maximium HFD during a training run should be ~ 9.0 - 15.0.
For example, a SBIG ST2000XM with RoboFocus and 130mm TMB F6 has a step size = 4 and 8 steps since it takes only 4 * 8 = 32 motor steps to go from in focus to out of focus with an HFD ~ 15.0.
For comparision, a QSI 583 with Optec TCF and 203mm TMB F7 has a step size = 150 and 15 steps since it takes 150 * 15 = 2250 motor steps to go from in focus to out of focus with an HFD ~ 11.5.
The step size and number of steps can easily be adjusted when training mode is used to create the proper range of test HFD values.

A detailed step-by-step example of the above training guide using an Atik 8300C CCD with 203mm TMB F9 APO and Optec TCF ASCOM focuser follows. The same steps are used with QSI, SBIG, and ASCOM color and mono CCD cameras.

An example of using auto focus after training can be found at CCD Auto Focus Use.

Step 1

From the main ImagesPlus menu open Camera | Atik | High Speed USB and connect to the camera.
From the main ImagesPlus menu open Focuser | ASCOM Focuser... and select your absolute position focuser.
Set exposure parameters on the Current Setting tab of ImagesPlus Atik camera control.
Set HFD as the focus metric and use Single Focus Test on the Focus tab of ImagesPlus Atik camera control.
Press the Release button on the Focus tab and a full resolution image is displayed.
Use the mouse to left-click and drag a rectangle around an isolated test star with HFD between ~1.7 and 4.0 for training or auto focus.
Press the Use Sub-image button and the subimage appears.

Select a subimage that contains an isolated test star with HFD between ~ 1.7 and 4.0.

Step 2

Left-click on the center of a star in the subimage to select it for training.
The lower right star is used since it is not overexposed and has HFD in the range ~ 1.7 - 4.0 when in focus.
Press the Done button and you are ready to start training or manual focusing.

Use a star that is not overexposed and has HFD in the range ~ 1.7 - 4.0 when in focus.

Step 3

Use the <,> buttons on the Focuser window to manually focus the star.
The star's HFD will have a minimum value when in focus.
Auto focus training starts with an in focus test star.

Use the <,> buttons on the Focuser window to manually focus the star. Training starts with an in focus star.

Step 4

Select Auto Focus on the Focus tab then press the Setup button. The Auto focus training and focus setup window appears.
Select Train then set Step Size = 150, Test Steps = 10, Average / Step = 5 on the setup window. Press Save and Done.
Auto focus training is now set to run starting at the in focus position and working inside and outside of focus using 10 test steps with 150 motor steps between test steps.
At each test step 5 images are taken and the excluded min/max average HFD of the test star is calculated..

In this case a training run is defined starting at focus with 10 test steps of 150 motor steps each inside and outside of focus . The excluded min/max average HFD of 5 images of the test star is computed at each step.

Step 5

Press the Release button on the Focus tab to start auto focus training.
The excluded min/max average of 5 images at 10 test steps inside of focus are computed first then the 10 test steps are run outside of focus.
The step size and test steps parameters can be adjusted if the HFD grows too large or there is not much change in HFD between test steps. To adjust the parameters press Cancel on the Focus tab and the focuser will return to its in focus start position. Now Press Setup and adjust the parmeters. Press Release to start training again.

Auto focus training as it appears at test postion 10 inside focus.

Step 6

Auto focus training mode has finished and displays the focus parameters.

Auto focus training mode has finished and displays the focus parameters.

Step 7

Press the Setup button on the Focus tab to display auto focus parameter details.
Check the computed HFD list and versify that the excluded min/max HFD values increase at regular rate inside of focus and decrease outside of focus.
The inside and outside slope values should be similar with the inside slope negative and outside slope positive.
Inside and outside correction values should also be similar and both positive.
If the inside and outside slope and correction values are very different than the step size and number of test steps do not allow a good sample of HFD focus values. You may need to increase or decrease the step size or test steps.
The focus parameters shown are typical and work well. Enter Atik 8300C 4x4 - 203mm TMB F9 as the description the press Save and Done.

Auto focus parameters from a proper training run.

Step 8

Auto focus is now ready to use and the parameters are saved for future use.
For a CCD the same bin size used during training is using for auto focus. For example, if you train with 4x4 bin the set the camera to 4x4 to use auto focus.
Auto focus with 4x4 is used to focus the CCD for use with 1x1, 2x2, 3x3, and 4x4 bin size.

Auto Focus is now ready to run using the Atik 8300C 4x4 - 203mm TMB F9 parameters. The camera will then be focused for any bin size from 1x1 to 4x4. An example of using auto focus after training can be found at CCD Auto Focus Use.

Select a subimage that contains an isolated test star with HFD between ~ 1.7 and 4.0.

Use a star that is not overexposed and has HFD in the range ~ 1.7 - 4.0 when in focus.

Use the <,> buttons on the Focuser window to manually focus the star. Training starts with an in focus star.

In this case a training run is defined starting at focus with 10 test steps of 150 motor steps each inside and outside of focus . The excluded min/max average HFD of 5 images of the test star is computed at each step.

Auto focus training as it appears at test postion 10 inside focus.

Auto focus training mode has finished and displays the focus parameters.

Auto focus parameters from a proper training run.

Auto Focus is now ready to run using the Atik 8300C 4x4 - 203mm TMB F9 parameters. The camera will then be focused for any bin size from 1x1 to 4x4. An example of using auto focus after training can be found at CCD Auto Focus Use.