Glen!
I received a PM from Dunk where he looked at explaining the reason behind having the focuser/corrector/camera (fcc) on the OTA orientated in the way you mentioned. In thinking about this between your posts, Dunk's PM and another experience I had with my Mak in seeking to balance it (yes,
that bloody Mak again...), I've figured out why it works!
It is a mechanical relationship between the OTA, where the weight is distributed along the OTA, and how the OTA is attached to the mount. Though it has nothing to do with it becoming a part of the counterweight axis as you said. The reason is a geometric weight distribution one.
When you look at a Newt, you have the primary mirror in line with the optical axis. The fcc is stuck on the side of the OTA, and no longer in line with the optical axis. The weight geometric axis of the OTA is off to one side running parallel (roughly) to the optical axis, but between the primary mirror and the fcc. Once the OTA is coupled to the mount, if the fcc is located above say the long axis of the dovetail plate, the weight along the scope is not in a symmetrically distributed, and it becomes impossible to balance the OTA.
Now, if the fcc is orientated below the dovetail plate, all of a sudden the weight distribution along the OTA becomes a whole lot more symmetrical as the weight axis is in line with the dovetail plate and most importantly crosses directly over the pivot point of the dec axis of the mount. The mechanics of the OTA balances out!
This is still not a perfectly balanced situation as the weight distribution axis is not exactly a balanced situation, and some fine tuning can be done by adding some small counterweights just above the fcc if this system is too light, or down at the primary mirror if the fcc system is heavier than the primary mirror system. The ideal situation would be having the weight distribution axis running directly through the dovetail plate, and then the entire OTA is balanced along the dec axis and attachment point!
Below is a little diagram I prepared to help explain things.
Glen, thank you so much for pointing me in the right direction here. I wasn't quite following what you were saying, and Dunk's PM helped me put all the pieces together!
I'll be looking at my own 8" f/4 Newt again with this new insight and look at ways to really fine tune its balance. It's also helped me figure out why I've also been struggling with balancing my
bloody Mak both on the NEQ6 and CPC mount. I now understand the weight mechanics at play!
Alex.
PS: Dunk! I nearly forgot to thank you too!!
