Focus Metric - how does it work /mean?

[quidnonn]

Greetings All!
Still struggling to get my allsky camera to work - but thanks to help here i believe I am making progress. Specific question here is about the focus metric - the documentation states that is helps in the focus but I'm not sure how. With itactivated, what does the number mean - are higher numbers a better focus? That said for the same field of view but an updated exposure the focus number can change even w/o and alteration of the focus ring. So when I am manually trying to set the focus (ZWO ASI224 camera) to infinity to get sharp star images Im not sure how to use this feature to its fullest. Thanks for any help!

[EricClaeys]

Did you look at the "How do I focus my allsky camera" question here.
It states that the higher the number the better the focus. Think of it as just a number. I believe it indicates the contrast in the image, and images of higher contrast are usually more in focus.

The documentation page also says the number can vary depending on the brightness, or in your case, the exposure time. Try to focus at a consistent exposure when the brightness isn't changing.

[quidnonn]

Thanks Eric!
I missed that - appreciate the link

[Croniac]

The experience I had is that the focus metric is helpful only if you don't have a better way to focus the camera. It does work as a rough idea if the image is improving, but it doesn't really help you get great focus.

What I did instead was remove my ZWO camera from its sealed allsky rig, and set it up clean on a tripod. Then using FireCapture (free online), a set it up doing short exposures at high gain (like half-second) on a dark night. With the image being displayed at full resolution full screen... Thus only a portion of the image visible on screen.

Make sure your area of view is near the center of the image... find a star that is not very bright, but not especially dim. Then focused till it was as sharp as I could get it. I then magnified further till individual pixels were visible, and I could see how many pixels my target star was registering on. With very fine adjustments to focus at that level you can see if you're using more or fewer pixels for the reference star, when it will not get smaller, thats as good a focus as you can get.

Once you put it back into your enclosure it will not look as good due to aberrations in the dome, etc. But that does not change anything with regard to focus, you are at the best possible focus and attempting to focus out the issues with the dome is a step backwards.

In its enclosure using that method I end up with this being the typical results...

[EricClaeys]

@Croniac I agree the focus metric may not get you the best focus, but for many people it's good enough. However, I just created an Issue to clarify this.
Thanks for the info.

[Croniac]

Hey Eric, I wasn't trying to call out any weakness in the Focus metric.. just pointing out that its a good secondary tool, but probably not a great primary focus tool. It works fine for what it is. I just thought it might be helpful to explain how to manually get a good focus.

[EricClaeys]

@Croniac, I viewed your post as providing helpful feedback, which was appreciated.

[EricClaeys]

Closing discussion - updated documentation in Point Release 4