This is a collage from a single 120 second frame assembled to represent the eight extremities of the 35mm frame of my SBIG STL-11000M plus the center.
Each box is full resolution (click on "Original" below)
There was no processing other than a single autodark subtraction.
The seeing was slightly worse than average and the FWHM of the whole frame was 4.4 pixels (at 0.64 arc seconds per pixel)

The stars display very slight elongation at the corners but otherwise the star shapes are very good across the frame.
NOTE: The mirror spacing was increased by 2mm from the optimum setting to achieve focus at 50% of the Atlas' range.