Everything that has been said here so far is correct BUT there are other ways to go about it.
The ultimate question is, what is your budget?
You are far better off getting an equatorial mount and using your current camera/lens combo. Doing this is the single biggest change that you can make. Taking 0.5s exposures with a 600mm lens is just not going to be what you're looking for. Yes, it'll allow you to zoom in on those star clusters but you'll never get a result you're really happy with.
Both of the images below were taken with unguided with what would be considered as a heavy duty Sky Watcher Star Adventurer.
http://www.astrobin.com/301645/
http://www.astrobin.com/301679/
The below shot was taken with a much better mount but I've had similar results with the HEQ5 that was used in the shots above.
http://www.astrobin.com/304742/
I do almost all of my wide field photography with an 8 year old mount with servo motors that hasn't been produced since around the time I bought it. It has a much higher weight capacity than the Star Adventurer BUT the Star Adventurer will for the most part do just as well with a DSLR and lens setup.
I'd suggest using the Star Adventurer with your 90mm and get your head around using that setup. When you want to upgrade your lens you can either get something along the lines of an ED80 or a 65mm ED Quadruplet F/6.5 (there are a few iterations of this) but your first step should be for an equatorial tracking mount.