This test is with the reducer mounted and with the CCD positioned 6" from the back of the OTA.
The mounting is with the AP SCT adapter plus a 2" long extension. The camera mounting is, starting with the camera, a 15mm spacer, the SBIG nosepiece plus a 2" UV/IR filter plus a 2" long extension tube to reach the Antares specified 79mm distance (+/- 2mm).

Note elongation of the stars, and not just in the corners.