Change brightness of 27" iMac (external display)

Hi,

Ok here is my set-up: I have a 2011 15" MBP and I hooked it up to my 2009 27" iMac to use it as an external display.

Now my question is:

Is there any way I can change the brightness of my iMac screen?
If I use the brightness control buttons on my MBP's keyboard or bluetooth keyboard, the brightness of my MBP changes but not that of my iMac.

Can anyone help me with this? It's annoying that my iMac's screen always at the maximum brightness, it's just to bright...

Thank you in advance!