First time I have actually been in a position to make a quantititve comparison with the offset of the corrector plate on a C11 SCT. These pictures will hopefully help other understand how critical the axial offset of this big lens in front of your primary is. In both test original conditions were as follow:
1_ Both shots were taken with an hyperstar so there is no secondary to compensate axial error via collimation.
2_ The hyperstar was not tilted or collimated in anyway. It sits flat on the corrector.
3_ Both photos were taken in the same conditions. Seeing, transparency, temperature, camera, etc...
In the first shot the corrector was shifted 1mm towards 2 o'clock facing the front. In the second shot the corrector was centered in the aperture. Both shots are taken in focus. The two small schematics show the corrector offset direction.
In the first case (offset) you can see stars flaring. The degradation in the picture is massive. The flaring in and out of focus will shift by 90 degrees, typical of astigmatism. The scope could not come in sharp focus. Even trying to collimate with the secondary in place would show oval airy patterns in and out of focus.
We are not talking about rotational shift here, just a 1mm axial shift. Rotational shift of the corrector would cause the telescope to not focus at all. Even a 5 degree rotational offset will make the SCT unusable.
I read that the corrector placement is critical before but these two shots illustrate it quite well.
I have a higher resolution pic
here. For those interested in the whole field there is a flicking slideshow between the two
here. [5.9MB]