There are 2 types of concerns when it comes to corrector plates (and I found about them the hard way). One, like mentioned above is when one removes the corrector plate to clean and doesn't put back in exactly the way it came out. Second is, after the corrector is back in, there are 4 little grub screws with silicon tips that hold the corrector in place, this will throw the secondary out of alignment and will make collimation all over the field impossible, if the secondary is not exactly squared with the primary and the visual back, your stars will look like little comets in the corners of the FOV and perfect in the center, to fix that there are a few visual tricks but they are not fast and simple and can be a very frustrating experience, the other option is to use the Hotech advanced collimator, one of the steps of the collimator procedure is exactly to check how centered your secondary really is, and even though the Hotech is very good and efficient, it is also quite frustrating. I'm glad Celestron took care of you, my advice is to leave the corrector alone, even if it looks a little dirty.
if i want perfect. I will need to reach deeper in my pockets for a HOTECH Advanced CT Laser Collimator. What a life 
