I'm a newbie. In the compatibility chart for GSO 2'' dual speed crayford focuser, it says either 96mm or 112mm model is compatible with Orion AstroView 120st. It seems 96mm is to install onto the collar, while the 112mm one is to install inside the tube directly. I wonder which one would be suggested and easier to install and use. I will probably also just remove the stock focuser and measure the tube myself. Thanks.