The best option for DSO imaging is prime focus however some scopes especially reflectors do not have enough inward focus travel to achieve this with a DSLR, the fix is to shift the primary mirror further up the tube which can work if the distance is small or use a barlow lens to move the focus point further out.
In my opinion a barlow lens works far better than an eyepiece for this but you do end up making the scope much slower, fine for Luna and Planetary imaging but not ideal for DSO but in some cases its the only option unless you just use the camera and a lens on its own.
Alan