Hi Dilshan
Backfocus is more an attribute of the scope/lens, not the camera (hence the name.. it's the "back" of the optical path).. it's usually called the flange focal distance for camera lenses. For scopes, just backfocus.
For example, the ED80 with an Orion field flattener has a backfocus of 55mm, meaning anything between the flattener and your sensor should take up 55mm*. I used to use this same scope/flattener and a DSLR as well.
The same DSLR with a lens requires about 44mm. Only thing that has changed is the lens/scope.
That said, in practical terms for a DSLR, there's not much difference between saying from the back of the lens (not telescope) to the sensor vs from the flange of the camera itself to the sensor, seeing as they are the same thing when the lens is connected.
*give or take. You'll likely need some spacers to get to the correct distance, usually a few mm away.
If you're using a flattener, add up all of the things that you plan to add between the back of your scope or flattener and the sensor. Your camera will consume 44mm of that total distance by itself, so you'll have 11mm to play with to add an OAG and a T2 adapter. Might be difficult unless there are super thin versions about.
Or you could look at something that does both OAG duty and connects to the camera:
https://www.teleskop-express.de/shop...he-T-ring.html
If you're not using a flattener (and you should
), then you'll have lots more room to play with.. I can't remember what the naked backfocus is for that scope (without a flattener) but I think it's >100mm.