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Star shape diagnosis

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Can anyone tell me what might be going on with my star shapes?  I think possibly I need to increase my back focus a bit but the corners are not uniform.  Perhaps some kind of tilt in there as well?

One debayered sub (180secs) attached.  Ca\non 800D, WO Z73ZS scope with WO 0.8 reducer with adjustable back focus set to recommended 1.8mm - total BF of 56.8mm.





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Looks very similar to my shots when i had focuser sag/collimation issues with my newtonian. Dont know if collimation can be an issue with your refractor but focuser sag or tilt somewhere in the camera-corrector-focuser area would look like this.

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I think there is a bit of tilt. The top left does not look too bad so that distance is probably about right. It might be the sensor or the optical train. Try turning the camera. If the image remains the same then it is the imaging train. If it moves it's more likely to be the camera.

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Thanks @ONIKKINEN and @Clarkey 

If it is tilt, I guess its a fraction of a millimetre?  I don't think I have room to fit a tilt adjuster but maybe some copper tape in the right place might help.

The weather for the next week is grim, so I'm going to have to be patient.

Thanks again.

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