G'day Bill,
I found that to get the QHY12 correctly spaced with the MPCC-3, a couple of 1mm spacers were required, on an 8" F/4. The internal sensor-to-glass distance is 20mm.
With the Orion Thin OAG, care needs to be taken with the guide camera shaft -- it is secured with two small grub screws that are made from a soft metal. They can wear out after a few uses, causing the shaft to move around when the camera is attached. Otherwise, it's a great little OAG
