Autofocus questions - Forums

Topic starter

New contributor

4 days ago

In the past, I've generally run an autofocus every 30 minutes (I don't have a temperature gauge on my rig). But I've recently switched to NINA and have used the autofocus on HFR change (default of 5% over 10 images). But no autofocus run was triggered during the entire night. The temperature change during the night was only 5.5 C (10 F), and I was using a Redcat 51 with an image scale of 3.1 arcsec/pixel. The images have come out fine, and I'm assuming there just wasn't a need for any autofocus runs.

Am I correct in assuming that given a more severe change in temperature or a different scope/resolution, an autofocus run might have been triggered?

Also, am I correct in believing that HFR variance is a better metric for triggering an autofocus run that temperature change alone, since HFR change is a direct measure of the need for refocusing?

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Tony Gondola

Frequent contributor

4 days ago

I don’t think HFR is a good guide for when to adjust focus because it can vary a lot as the seeing changes through the night. Temp at the Objective would be a better guide. I think that once you know your rig really well, you’ll know how big a temp shift needs to be to push you out of the CFZ. If you study your HFR log you’ll see the trends. Seeing usually but not always will be up and down. Focus shift do to temp will be a constant trend in the data. It’s pretty unmistakable when you see it.

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Tom Marsala

4 days ago

I agree with Tony here. Especially in my large newt where seeing changes and wind conditions are less than optional, my rig would focus almost every five minutes when I was working with just HFR. I finally settled on just temperature as the main variable when using NINA.
Tom

andrea tasselli

Frequent contributor

4 days ago

Dave Wilton:
Also, am I correct in believing that HFR variance is a better metric for triggering an autofocus run that temperature change alone, since HFR change is a direct measure of the need for refocusing?

*For small apertures, short focal length HFR is a good metric to trigger autofocus, and the core of the temperature sensitivity might be more on lens' temperature than air temperature.

Tobiasz

4 days ago

Refocus on % HFR change does an autofocus when your images are already blurry while a refocus on temperature change tries to prevent your images getting blurry.

Even my steel RC with f/8 lost its best focus when the temperature moved around 1°C to 1.2°C.

I would recommend setting two triggers:

Autofocus every x min

Autofocus on x°C temperature change

Nina will reset the “autofocus every x min” timer if an autofocus already happened because of the temperature. This way you can make sure to have the best focus all the time.

But the temperature probe needs to be accurate, if its temperature jumps around to quickly you will never stop auto-focusing.

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