[Shiraz (Ray)]

should be a really good scope/camera combination.

at prime focus, you will use the scope as the lens for the camera, so you will need to run the camera in manual, and set all parameters by hand. Focusing will be manual by way of the scope focuser.

Its not clear how much of this you have already done, but to check if you can get focus, focus the scope through an eyepiece at something a long way distant (you need something a few km away to keep this test simple) from inside a darkened house (through an open door/window, not through glass). Measure the distance between the top of the fixed part of the focuser and the top surface of the eyepiece adapter. Then take out the 1.25 eyepiece adapter and wind the focuser in as far as possible - measure the distance from the top of the fixed part of the focuser to the top surface of the moving part. Then hold the camera (without lens) up hard against the end of the focuser tube and adjust the focuser outwards. If you can get a good image, you may be in with a chance of getting it to work without any scope modifications. Measure how far out you have wound the focuser to get focus and if that distance is greater than the height of the camera adapter, you will be OK.

If you cannot get focus, the CCD in the camera is too far away from the front of the camera and you may need to move the main scope mirror up the tube a bit to compensate - longer bolts, spacers and springs as Brent suggests if you don't want to modify the tube, but be very careful that the longer bolts cannot possibly touch the mirror. A lower profile focuser might also give you enough extra focus, without moving the mirror, if you can get get to focus, but don't have enough focuser travel to fit in the adapter. With an f5 system, you will probably need an MPCC to reduce coma at the edges of the image - really depends how fussy you are. The MPCC will also help by adding 10mm extra back focus, but it adds extra problems of its own (getting the right spacing for your camera). To make the best decision, you really need some measure of how much extra focus your system needs. If you cant get it in focus in the above test, post the focuser distances you measured and someone will help you work it out. Would also be helpful to know the distance from the front face of the lens mounting ring (lens off) to the CCD chip (should be visible looking into the camera from the lens mounting) - just guess by eye, don't put anything inside the camera body.

If you want some idea of what you will get at prime focus, put in a 15-20mm eyepiece, fix the 18mm lens on the camera (set it at infinity in manual focus mode) and hold the camera+lens in front of the eyepiece (in the darkened room looking at something distant through the door/window as before). You should be able to focus an image using the scope focuser - the eyepiece and camera lens act as transfer optics to get the image formed by the scope onto the CCD. This method puts lots of glass components in the imaging train, so it is not the best way to go, but it will at least show you what you can expect to see when you get it going in prime focus.