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The setup for autofocus in NINA is pretty straight forward, and most of the settings can be left as default. It's best to start simple and only tweak more advanced settings once the basics are working. At the start, the only parameters that need adjusting are exposure time, "Auto focus initial offset steps" and "Auto focus step size".
It's important to start at or close to focus. You can do this manually by taking short exposures and changing focus position until the HFR is optimum. "Auto focus initial offset steps" is just the number of exposures the system will take on either side of the start position (I use 4). The best autofocus run will result when the first and last exposures (i.e. the ones most out of focus) give an HFR approximately 4 times the HFR at focus. For example, assume at best HFR the focuser is in position 1000 and the HFR is 2.0. Move the focus position out (or in, it doesn't matter) until the HFR is around 8.0 (assume this is focus position 1200). Then, with initial offset steps set to 4, the step size will be 50 i.e (1200 - 1000)/4. By the way, 4x the HFR is a rule of thumb. Don't get caught up trying to be exact!
Exposure time should be set long enough so that enough stars are visible but not so long that waiting for 9 exposures takes forever! For a one shot colour camera or broadband filters I think 2-5 seconds should be fine. I set narrow band filter exposures to about 10s.
Hope that helps...
Kevin
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