As found tonight, using William Optics acrylic Bahtinov mask.
Note the t2-extension tubes required to make the Tele Vue 14mm Radian eyepiece in the finder port of the TS-Optics OAG/flip mirror par-focal with the camera. This number of extension tubes means the eyepiece has to be pulled out slightly but when I tried next size up on one of extension tubes then eyepiece could not obtain par-focality.
