To go really deep, I favor taking subexposures that are 5 minutes or longer with my 11 RASA (620mm FL) on a Losmandy Titan. I have two guide scopes: a 45 Borg with a 325mm FL and a 30 QHY Mini Guide Scope with a 130mm FL. Surprisingly, the smaller QHY Mini GUide scope with a shorter focal length consistently yields far better guiding results than the larger guide scope. I do not know why the PHD guiding graphs are much tighter with the shorter focal length, but I am ordering another QHY Mini Guide Scope for my other setups. Besides yielding excellent guiding results, I also love how the scope is extremely light, has its own mounting rings and low base that does nothing to add to the inertial mass of my imaging setup unlike other guide scopes that need heavy rings, base plates, and sit high. The scope holds focus well once everything is tightened down, and the rings/base have no slop.
The product arrived in working order. This interfaced with the Orion RC 6" well and reduced the weight and moments of inertia.