Off-Centre image circle on Newtonian

Sky-Watcher Explorer 150PDS
5 replies•75 views
John Walsh avatar
John Walsh avatar

Hey all.

I’ve included to flat contour plots of my master flats from 2 different camera rotation angles.

You can see that the image circle is not centred. The plot also seems to show oval illumination in the centre rather than circular.

I’m just wondering if this is an issue with the secondary’s position in relation to the focuser or something else?

Scope: 150PDS

Camera: 2600MC Pro

Stock focuser, no EAF.

I have not tried to address it yet as the flats work fine on the image but I would like to try to fix it during the summer.

Just looking for some advice from those with more experience than myself

Many thanks

John

📷 Screenshot 2026-04-27 at 06.56.44.pngScreenshot 2026-04-27 at 06.56.44.png📷 Screenshot 2026-04-27 at 07.00.39.pngScreenshot 2026-04-27 at 07.00.39.png

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andrea tasselli avatar
andrea tasselli avatar

Don’t bother. If you really have to, check without a CC where the center of field lies and offset the secondary to center it, if you can.

Alex Nicholas avatar
Alex Nicholas avatar

This indicates your secondary is not centered in the focuser. .. The direction of the de-centering can be inferred by the orientation of the camera youre testing this on.

Are you seeing any tilt in your images?

Helpful Concise Engaging
Alex Nicholas avatar
Alex Nicholas avatar

As Andrea said, this is a pretty minor and if your collimation is good, I wouldn’t bother adjusting this. Unless your images are showing distinct amounts of tilt…

Well written Concise
John Walsh avatar
John Walsh avatar

Alex Nicholas · Apr 27, 2026, 08:41 AM

This indicates your secondary is not centered in the focuser. .. The direction of the de-centering can be inferred by the orientation of the camera youre testing this on.

Are you seeing any tilt in your images?

Thanks Andrea & Alex. Much appreciated.

I can’t say that I have seen any evidence of tilt to be honest and collimation is good.
Might just leave well enough alone like you advise so.

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Tony Gondola avatar
Tony Gondola avatar

John Walsh · Apr 27, 2026, 06:12 AM

flats work fine on the image

That’s the key right there….