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Askar FMA180 PRO diffraction pattern and autofocus

Askar FMA180Pro Askar FMA180 Auto-Focusing
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Olivier G avatar
Olivier G avatar

Hello, I have some issues with autofocus on my Askar FMA180 PRO pezval refractor.

The star shape intra and extra focal are very different, and a small point appears in the center of the diffaction pattern.

📷 autofocus steps.jpgautofocus steps.jpg

The autofocus engine is tricked by that point, and sometimes converges at an incorrect focus point.

📷 autofocus curve.pngautofocus curve.png

I optimized the BFD with some spacers, and it seem to me that I have the correct one. In focus I have very good star shape throughout the frame.

Can any other fellow FMA180 owner confirm the same out-of-focus pattern?

Is there any parameter in NINA or hocus focus that can help me get a more robust result?

Thank you! Olivier

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Tony Gondola avatar
Tony Gondola avatar

You might consider reducing the number of steps or the step size so on the extreme ends of the curve the stars aren’t as far out of focus. That said, this looks more like a backlash issue. Notice that the system didn’t have trouble with the out of focus images on the left side of the curve.

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Dan Kokinda avatar
Dan Kokinda avatar

I can tell you that my curve looks similar to yours only reversed left to right. I have never looked that closely at the individual focus step images as my auto-focus routine invariably produces exceptionally sharp images. You can see a couple of final images with this scope under my profile.

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Olivier G avatar
Olivier G avatar

Tony Gondola · Jul 5, 2026, 03:27 PM

You might consider reducing the number of steps or the step size so on the extreme ends of the curve the stars aren’t as far out of focus. That said, this looks more like a backlash issue. Notice that the system didn’t have trouble with the out of focus images on the left side of the curve.

Thank you Tony for the answer. I will play a little bit with the steps parameters. I’m very confident it is not a backlash issue, though.

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Olivier G avatar
Olivier G avatar

Dan Kokinda · Jul 5, 2026, 03:35 PM

I can tell you that my curve looks similar to yours only reversed left to right. I have never looked that closely at the individual focus step images as my auto-focus routine invariably produces exceptionally sharp images. You can see a couple of final images with this scope under my profile.

Thank you Dan. Glad to know that the curves are similar in your scope. Indeed the image is very sharp when correctly focused, it really is a fine little scope! You have some very nice images, congratulations!

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Werner Stumpferl avatar
Werner Stumpferl avatar

My curve is similar to yours but the sharpness of the image is not affected as far the focuspoint is the lowest in the curve. Also in my case it has nothing to do with backlash, I have tested a lot of steps without any change. So don´t we worry.

Olivier G avatar
Olivier G avatar

Thank you Werner.

I have just realized that I was using a wrong timing belt on my focuser. The telescope is designed for a MXL timing belt (pitch 2,032 mm) and I was using a GT2 profile (pitch 2mm). This was causing the belt to sit incorrectly on the ring and sometimes “jump” during autofocus.

A new belt is already on its way!

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