hi
yep a smaller secondary is not a good idea. I'm pretty sure the size of the secondary won't change the distance to the focuser from the primary. Just how much it is illuminated.
I'd suggest increasing the front aperture until that error disappears. working with metric units help.
your dslr should have a white line on the body that shows the position of the sensor. so best to calculate distance required with that as the reference.
in my case, I calculated the positions as much as possible, and then i built a jig with the primary fixed and the upper assembly adjustable.
I then went to an open area from where I had visibility of some buildings and trees two km's away. I put in the dslr and racked all the way in and then out about 5mm, focused on the distant object and marked the position of my UTA.
the qhy8 ccd focal point was well out. so I'm able to use either the dslr or the ccd on the same scope without moving the primary.
Have a read through David Fitz's CF 12" build in the ATM section. he's achieved some really nice results mainly due to the rigidity.
http://www.iceinspace.com.au/forum/s...t=64882&page=4