Perhaps it's a bit late to be describing what the problem was, but since starting the on-sky testing we've dug out old images & it's worth looking back... The most serious contributor to the SALT IQ nightmare was the focus gradient, which meant that only a small portion of the 8 arcmin field could be kept in good focus at any given time. By adjusting the telescope's focus, different parts of the field could be optimised, but always at the expense of other regions. This in itself was unacceptable, but the focus gradient also exposed a variety of other horrors, including mutilated stars in regions that were out of focus & severe astigmatism. Finally, there was the hideous notion that all of these woes somehow vary unpredictably due to a combination of factors... Below is a typical bad IQ image, obtained with the Apogee camera in late September 2007. Best focus is in the top right corner, the bottom left corner shows the mangled stars & overall the whole lot's just a Mess!
Fast forward through years of extensive detective work, the formulation of an extremely thorough repair plan, the 16 months it took to execute the plan & then wait for mediocre observing condtions to get the image below... Uniform IQ over the entire field of view - The Focus Gradient Is Gone!! :) :) :) But the good images aren't stable over time & one can see strange things going on when you take the telescope out of focus - the doubled images that we've dreaded all along!
That's where the auto-collimator instability came in & after fixing up the mount & then attaching the whole lot to the steel collar of the SAC, we obtained really beautiful images last night! Still no focus gradient & after optimising the auto-collimator offsets, they didn't need to be tweaked throughout the night - the images weren't doubled or distorted any more :)
What remains is a small amount of astigmatism, which we now know has to reside within the primary mirror (since we've fully re-aligned the SAC & tested the surface figures of its mirrors & found them all to be in great shape). So that's the next step, but we're down to the finishing touches now! :)