Thank you all, but "luminary" is a bit much, IMHO, unless you're referring to the effect of the bright Southern California sun bouncing off my bald head, temporarily blinding those around me
Anyway, it's hard for those getting their guiding rigs going to underestimate the role that flex can play in making us tear our hair out (did I mention I was bald?). A few weeks ago, I was looking at a new-to-me 8" f/5 Antares (Synta) Newt and the new "wsg" off-axis guide setup for my QSI camera. I ran some tests using my current guide rig (the Borg XY setup -- great, great setup) and the onboard "wsg", looking at the post-guiding error (what Nebulosity reports it had to do to align the frames).
With the "wsg", I averaged 0.14 pixels of error across a 25 minute run (this is error in the main image, so where it really counts). I could stack without any alignment and have no issues at all (using whole-pixel alignment, no frames were shifted at all).
Without and using a separate guide scope, the error was horrible.
I was drifting at almost 2 pixels per minute!
Why? Does my guide scope suck? Nope, the Borg is a rock (previously tested as equal to at least 0.95 rocks). The focuser and the main tube itself flex a good bit under the load of this heavy camera. Move the camera around and you can watch the tube flex. Think that'll have an effect on your collimation and your guiding accuracy? You bet...
Anyway, that's why I like this HoTech setup. The coupling of the scope to the focuser is one place I've had to fight this (I had a really bad time of it on a TMB 105-LW I used to have). It's such a simple idea and it just plain works. That source of flex is just taken out of the equation (and you're left to fight the other sources...)
Craig