There are 3 mirror in the machine, 2 very small mirrors (called the Galvo Mirror), about 5mm X 7mm that each of them connect to a very accurate motor, The last mirror (Main Mirror) is a very large one located below the glass of the machine, you can see it if you remove the LT tank , its almost the same diameter as the glass but is placed with angle to the glass.
I haven’t took a picture of it so here is the picture from the cleaning manual:
It was not much of a work and it was dusty for sure, I could see the dust on the large mirror. I’ve took advantage on the process and also clean the glass from below, it was much more easier then to disassemble it from the top. and some other plastic and metal parts that was covered by the aluminium cover.