What camera are you planning to attach to the ST80 ?
Maybe you aren't, I'm not clear what you want the ST80 for.
Images of the ST80 show it with a 90 degree or a 45 degree Star Diagonal attached to the focuser.
This is an extra piece of glass that Imagers discard - with Imaging, every piece of glass in the system cuts down the transmitted light a small amount.
To compensate for the removed Star Diagonal, you need an Extension the same light-path length as the missing Diagonal.
Without it you may not be able to rack the focuser out enough to get focus.
For Visual use you don't need the Extension, just use the Diagonal and eyepieces.