I have been testing an 8" Orion DOB, a 1978 Meade 856 8" f6 Newt, a 10" Antares DOB and a 12" collapsible Skywatcher DOB with the EasyTester on the brightest star I could see. All have parabolic mirrors. I am getting very consistent straight lines as you can see from the photos below (taken with my iPhone held to the eyepiece so some vignetting and the atmosphere was very turbulent).
Firstly, I have read that the lines for a parabolic mirror should not be parallel whereas they should be for a spherical mirror and have also read the exact opposite (the insructions for the EasyTester). I'm confused...
Secondly the patterns I am getting on either side of focus are significantly different from each other. Any idea what is going on?