Quite opportune this Russel as I'm setting up an ASI6200 myself at the moment so we can share some stories along the way
. In my case I have an FSQ106 that I have purchased as pre loved and initial tests show a combination of tilt and possibly collimation. I won't be able to diagnose any further until these pesky clouds go away.
Nevertheless, looking at your image on Flickr. It won't let me download a copy so difficult to look too closely. If you could provide a link to a raw fits file I can have a closer look.
It is clear though from what I can see on the Flickr image that there is radial elongation on the top left and to a slightly lesser extent bottom left. If it were incorrect spacing from the reducer to the image plane on the camera I would expect concentric or radial stars in each corner so maybe tilt is worth looking at.
Did you install the ZWO tilt adjuster at the camera end or the OAG end? I'm not using the ZWO tilt adjuster so cannot comment on its use but I have seen reports that it is difficult to adjust.
A question about the M54->M48 adapter. Does this mean that you are using the TSRED379-130 Reducer/Corrector with the M48 connection on the camera side? If yes, I see from the Teleskop site that the backfocus distance is 55mm which is exactly what the ASI6200 image train delivers with the tilt adjuster included. This means that you really don't have any room for tolerance in the backfocus from the reducer to camera.
If the M54-M48 0mm adapter that comes with the camera is not doing the job then the only solution that I can think off is to remove the ZWO tilt adjuster and have an adapter made up which is M54-M48 @ 5mm length. This would give you a more robust connection which is going to be required anyway even though you will be losing the capability to adjust for tilt. Alternatively, have the adapter made to a 3mm length and purchase a couple of 48mm spacers to allow for some trial and error in the backfocus.
So thats a few thoughts from my side Russel. I will provide an update on my setup when I get back under the stars with my new camera.
Clear skies,
Rodney