My C5/PST setup has internal reflection issue, the problem is from the blocking filter, it reflects light back to the secondary. It seems like it's hard to correct in a SCT kind of system. A refractor is a must, therefore.
From the past experiments, I know we need at least f/7, and f/8-f/10 would be even better for the PST etalon module.
A bigger refractor will also be nice to fully exploit the potential of my Herschel Wedge, and therefore also the Baader CaK.
At the same time, I believe the C5 would be the maximum for my TG-SP II for planets, however, I found it's not the case, it's simply too heavy, I will need my LXD55 in that case. So, a C5 is no longer the choice.
A ~4" to 5" refractor would be nice, and to keep my LXD55 happy, I guess that I'll need to stay below around 12lb. That would be fine with my Unistar Light mount for visual observation as well.