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I guess you are right with the focuser thats why I countered that with utilizing screw down fittings all the way down to the focuser and a FT 3" focuser because i knew that the standard Skywatcher 2" sucked big ones and with the addition of a 3" OAG the QHY9 mono plus associated filters in the filter wheel there is not a hope in hell the Skywatcher can hold onto that with any reasonable accuracy.
Secondly if your moving your collimation that much from when your autocollimator out to when you put your camera in start fixing! When i do my collimation i have a reference screw which i always screw down first and then in a clockwise fasion tighten down the remaining two this makes sure that the camera is sitting virtually in the same spot as the auto collimator and hence my collimation routine is still valid.
The other thing that you have to remember that its well and good to collimate to your focuser if you have a stationary focuser but if you want to frame your object to take full advantage of your sensor you are screwed big time because you will have to not only have to do a focus routine on every object you will have to collimate on every object which is not really feasable and in the case of my FT it has a 360 degree rotatable focuser which holds focus. So at the end of the day you want to get your collimation perfect so to speak that way you have the best flexibility.
You can see from my images that i have round stars across the FOV admittedly this isn't a 11k like yours but then again if im getting pin point stars to the edge of FOV now there will be no difference if i increase the sensor apart from if there is any residual error it will be more evident but still under the seeing celing.
In relation to the tilt of the ccd sensor, if it has tilt then utilizing the cats eye gear will give you a big piece of mind and a base line that the imaging plane isn't tilted. this will then show up inadiquacies of other components and allow you to fix them as you go.
I guess at the end of the day it all comes down to your equipment, if your trying to get ultra fine and precise alignment using the bottom of the line gear you will always struggle to push 5hi7 up the hill. on the other hand if you get the top of the line gear... it just works seemlessly. Most of us spend our lives somewhere inbetween and as you say you do what works for you but for me I am a tinkerer and i wont stop till i have figured out how to solve my issues not band aid them for the long term.
Brendan.
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