Moonlite V2 sinlge motor Focuser

[TrevorW]

I have the focuser working under NINA but need some guidance on how to set up autofocusing with this version of Moonlite any help would be appreciated

[kosborn (Kevin)]

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

[TrevorW]

Cheers Kevin do you have to move off target and pick a star to get autofocus to work as it should

[kosborn (Kevin)]

Originally Posted by TrevorW

Cheers Kevin do you have to move off target and pick a star to get autofocus to work as it should

It's best to perform auto focus on the target field of view because that's what you will be imaging. The HFR (half flux radius) calculations that NINA does are on multiple stars within the field of view, not just a single star. If my start position is way out of focus then NINA may not be able to calculate a HFR because the stars don't look like stars. In that case I look at individual stars while doing a manual focus to make sure I am heading in the right direction. Once I get close I then just watch NINA's HFR number instead of what the stars look like.

If you are still having difficulty then I recommend this video. The technique he describes is a little different to mine but the explanation is very clear and he gives a good overview of the auto focus routine and what is happening in the background. He also describes how to deal with backlash in the focuser which is something I had a problem with, and which was the main reason I changed from SGPro to NINA.

Kevin