Get rid of the diagonal and get a
prime focus adapter. It will screw directly into your T ring adapter and go straight into the back of your scope. (This is assuming of course you have a 2"visual back on your telescope, otherwise you will need a T - 1.25" prime focus adapter but will get vignetting in your shots. I'd suggest getting a 2" visual back adapter) You should consider a focal reducer as well. It will make life a lot easier shortening your focal length.
The suggestion of setting up and trialing during the day is an excellent one. And don't be surprised if you have to screw your focuser in or out half a dozen to a dozen turns. You might even find it useful to wind your focuser all the way in and then all the way out, this will spread the grease on the shaft inside and help reduce image shift during focusing. Once you are all the way in or out you can start your focusing run on a tree or a building in the distance, something that is really obvious and has good contrast of tones. Once you have focused your camera take it off, put in your diagonal and eyepiece, then count your focus turns til it comes to focus (I'm betting you need to turn your focus knob clockwise to achieve this). Remember the directions and number of turns for future reference. When you then setup at night select as bright a star you can see and then using your remembered number of turns and direction bring it to focus.
BTW How's your polar alignment? It needs to be very good, not just good. You will need to learn to drift align if you don't already know how to. Your mount looks fairly light to be trying to image through an 8" celestron. Maybe consider an ED80 to image through for Deep sky stuff and keep the Celestron with a webcam for planetary imaging.