Backspacing calculations with 90-degree diagonal inline.

DobieAcres

Greetings, everyone. I am running the 1294MC off the back of a Celestron SE8 with a focal reducer and 2" 90-degree diagonal. Using a bahtinov mask I can get decent focus but I want to know where the 55mm backspacing measuring should start: 1) back of OTA; back of focal reducer; 3) start of diagonal (plus the "turn").... TIA, Mike

Tech@ZWO

From the back of the reducer to the sensor, not including the reducer.