Q: imac being used as second screen wont stay as screen after sleep

I've connected my macbookpro to my imac and want to use it as a second screen.
I've then taken the bluetooth mouse and keyboard to control the entire system via my macbook pro
when the system sleeps, the imac does not wake up as my MBP does
so when i manually wake it up with the power buttom (it does not go through startup)
its back to being an imac except now i can press command f2 to do the screen becuase there is no keyboard associated with it

How do i keep my MBP in command?