How to clean dust on front element of a SEL50F18?

I have just done cleaning the front element of my SEL50F18. I do not know how this brown dust snug into my lens.

So, what you need to do is to remove the front black ring ( refer below picture item no.1). There's a simple double tape underneath it, so no need to worry on breaking any plastic.Then unscrew the three screw (no.2).  Then remove this ring (no.3).You'll find a simple soft black seal underneath this ring not secured by anything. After that unscrew another 3 screw (no.4) of the front element and walla, it's better to use a forceps to assist in removing the front element. Please remove this element with care. There are few tiny thin shim (check the last picture) underneath this element and it is easily displace, damage and tedious to rearrange.

The tiny shim under the first element.