Cone Error? Or something else?

ZWO ASIAIR Sky-Watcher EQ6-R Pro
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John Walsh avatar
John Walsh avatar

Hi all,

I was imaging the other night right next to Polaris (Declination 88 degrees).

I discovered after the Meridian flip that the image was no longer centred at all or even in the field of view.

Cone Error is the culprit I assume.
I have noticed slight frame rotation after meridian flips before, but in this case, once I got the target manually centred after the flip, the degree of rotation was extreme.

So much so that I had to crop an insane amount out of the stacked image.

I’ve attached a screenshot of the final stacked image so you can see how much rotation was involved.

The only thing is, I would have thought that the diffraction spikes would be misaligned with cone error and they are not.
I hope the axes on the mount are not the issue but I don’t know how to go about checking definively in any event.

Would appreciate any input on this as I’m sure there are many more experienced than me that might have encountered something similar.

Gear in question:

2600MC Pro

150PDS Newt

EQ6R-Pro

ASIAIR

📷 Screenshot 2026-04-15 at 14.47.06.jpgScreenshot 2026-04-15 at 14.47.06.jpg

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Tony Gondola avatar
Tony Gondola avatar

I doubt that the mount could be off that much. When I have seen this it’s usually do to something in the imaging train getting loose and rotating.

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Lynn K avatar
Lynn K avatar

When you say “Cone Error", I am assuming you mean cone error in the mount's orthogonality. Un-orthogonality in a mount will cause bad star alignment when the mount is flipped. You can test this by simply looking through an eyepiece visually after the mount is flipped. Plate Solving usually solves this problem and re-adjust the star alignment.

Mount Un-orthogonality should not cause rotation. For that to happen there needs to be rotation in the optical axis. More likely, something is loose in the optical train and the weight distribution caused by a meridian flip is causing the camera or something in the optical axis to rotate.

Lynn K.

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John Walsh avatar
John Walsh avatar

Lynn K · Apr 15, 2026, 03:23 PM

When you say “Cone Error", I am assuming you mean cone error in the mount's orthogonality. Un-orthogonality in a mount will cause bad star alignment when the mount is flipped. You can test this by simply looking through an eyepiece visually after the mount is flipped. Plate Solving usually solves this problem and re-adjust the star alignment.

Mount Un-orthogonality should not cause rotation. For that to happen there needs to be rotation in the optical axis. More likely, something is loose in the optical train and the weight distribution caused by a meridian flip is causing the camera or something in the optical axis to rotate.

Lynn K.

Thanks Lynn & Tony.

I don’t believe there was any rotation in the image train as the Pre and Post meridian flip images were both perfectly flat-corrected using the same flats as I have been using for many months now(as I have left the camera rotation unchanged).

I thought that cone-error could cause rotation when very close to the NCP but could be mistaken

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Tony Gondola avatar
Tony Gondola avatar

If this is something you are seeing everytime you filp and it’s not a one off you can always run this to measure and correct.

https://www.sharpcap.co.uk/conesharp

John Walsh avatar
John Walsh avatar

Tony Gondola · Apr 15, 2026, 07:04 PM

If this is something you are seeing everytime you filp and it’s not a one off you can always run this to measure and correct.

https://www.sharpcap.co.uk/conesharp

Thanks Tony. I see it frequently alright but the effect is very minimal. Imaging so close to the NCP I think exaggerated the effect. Will check that out thank!

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Freestar8n avatar
Freestar8n avatar

Cone error can indeed result in field rotation when imaging close to the pole. This is because, near the pole, the dec. axis doesn’t move cleanly just in dec. As a result, pointing near the pole involves shifts in the dec. and ra axes to point at a specific target, and these shifts will result in field rotation. The mount can still point accurately if it has a good mount model and can compensate for this effect, but the field will need to rotate in order to point the same place after a flip.

Note that with cone error it is physically impossible to point directly at the pole. The cone of sky around the pole is completely unreachable, in terms of direct pointing.

This won’t be an issue with normal tracking as long as the mount is polar aligned, but a meridian flip would involve some field rotation.

The spider vanes should also show rotation (after alignment, because they are fixed in orientation relative to the sensor) and it’s odd that doesn’t show here in the stack, but my guess is there are fewer exposures after the flip and they were sigma rejected. A raw stack without rejection might show the rotated vanes, or just do a direct comparison of an exposure before and after meridian flip.

Frank

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