The distance between the reducer and the camera plane is critical. My telescope is a Takahashi TOA-130. I will be matching it with the TOA reducer and SBIG-STT 8300M camera with an FW8G-STT filter wheel.
My Imaging train
TOA-130 with The TOA-RF reducer backfocus
The first key number is calculating the backfocus distance. (more here)
The number I am looking for is 83.7mm, which is the optimal distance from the reducer to the CCD.
SBIG STT8300M and FW8G-STT Spec backfocus
For details go here
I am using 3mm filters so the back focus distance according to the chart is 52.5mm. SBIG technical support advised me to add 3.7mm for the mounting plate, The camera backfocus distance 56.2mm. A spacer is required between the focal reducer and the CCD.
Spacer length = 83.7- 56.2 = 27.5mm spacer.
To build one go here.