maplearn seriously distorted towards the edge
The regular C14 has an image circle of 20.46mm. The ASI2600 sensor diagonal is 26mm.
(The EdgeHD have an image circle of 42mm.)
I assume you have already checked these documents before purchasing a C14, but here they are anyway:
https://www.celestron.com/blogs/knowledgebase/what-is-the-maximum-flat-image-circle-or-radius-in-the-regular-non-edgehd-c14
https://www.celestron.com/blogs/knowledgebase/what-is-the-optimized-image-circle-for-the-edgehd-otas-and-reducers
Notice the first document above states that without a reducer, the C14 will still produce coma even inside the 20mm image circle. The C14 without a flattener or reducer is really meant for at best visual work, or imaging with very small sensors.
Without a reducer, you will need to crop the image substantially if you want to use an ASI2600.
An SCT is a compound optical system, and back focus is also critical even when you are not using a reducer. The back focus of the C14 is 5.975 inches from the metal back of the OTA.
https://www.celestron.com/blogs/knowledgebase/what-are-the-optimum-back-focus-distances-for-celestron-scts
If you want a flatter field (more or less round stars all the way to the edge of the image circle), you will need to use a field flattener or a reducer. The Starizona reducer for an SCT should give you a flat image circle of 27mm.
If you are currently already using a reducer (or field flattener), your plate shows stars that are enlarged in the radial direction -- this means that your back focus is too short. You need to space the camera at a greater distance from the reducer than you have now. Check with the manufacturer of your reducer to find what the correct back focus should be, subtract the flange focus of your camera from that (the ASI2600 flange focus is 17.5mm) and that would be the precise spacer you need to connect your camera to the reducer. If you are using a filter, be sure to also add the 0.5mm to 1mm contribution from the filter glass thickness (to factor in difference in speed of light in free space and speed of light through the filter glass -- usually around an extra of 50% of the glass thickness, since refraction index of glass is about 1.5).
If the elongation looks like stars at the edge of the field are rotating around the center, the back focus is too long. So, just keep adding spacers until stars at the edge just starts to look like they are rotating around the center, and then back off. This might give you something that , while not technically good, may be good enough for your eyes.
preciseparts.com can fabricate the exact spacer that you need. Some telescopes, like the Takahashi Petzval OTAs require back focus of sub-millimeter accuracy to take full advantage of their optics. Precise Parts' lead times nowadays is about a month, so you might want to experiment with standard spacers for the time being, to get a reasonable compromise.
Chen