This adapter allows you to convert the rear cell of the ETX Maksutov telescopes to accept SCT accesories, such as visual backs and focal reducers.
I have been using my ETX 90 as a grab and go scope, and I wanted to put an additional diagonal on the back so I could use 2 eyepieces at a time.
This adapter is essential to my plans.
Luckily.. Scopestuff will soon offer their own version of this adapter. All will be right in ETX land.