I worked on the corrector again last night for about an hour. It is closer but still has a long way to go. I don't think this thread is going cause anyone to pull out a corrector from their SCT and just polish away and make a mess of it. I have been teaching mirror making for 35 years now and that doesn't happen with newtonian mirrors so I doubt it will happen with correctors. As John Dobson's said "any errors you polish in, you can polish out" So unless you drop the corrector you can fix it.
When it comes to double pass autocollimation the result you see for the errors for the complete telescope are exactly what you see when your testing a spherical mirror. A telescope in theory takes parallel light and brings it to perfect focus. The wave form it produces is spherical ie all the light comes to focus at the same point. It is same as testing a spherical mirror at the radius of curvature which will cause all the light to come to perfect focus. So if you look in any good book on making mirrors they will show you what these errors look like when testing a spherical mirror. These errors will look the same and be the same when you test a complete telescope via double pass autocollimation. So a turned edge will look the same, or undercorrection or overcorrection will look the same. If you what the errors are then you understand what you need to do fix them.
As for collimation and misunderstanding the results, that can't happen. Misalignment of the optics is very easy to detect because you don't get a symmetrical pattern left to right and up and down. So anyone that has any experience in making optics will know this and anyone that is trying this method out for the first time I hope would do their homework on how it works and what you should see.
Check out the Star Test thread in the ATM forum that is active right now. http://www.cloudynig...-test-question/ The author started out questioning if he was doing the star test correctly and a number of us told him how to do double pass autocollimation using a pan of oil. He went from believe that he had a good lens to determine very quickly that his lens had a major issue and was able to get a replacement . So within a short time he went from little understand to knowing how to do this test.