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This is a 2 part question regarding Auto Guiding
1. I have never used guiding before and I would like to start shooting DSO using guiding. How important is it that the guide scope be aligned exactly to the main scope?
2. I am going to use my ZWO ASI120MC or one of my webcams. None of my cameras for guiding have the ST-4 port. Can I run the ST-4 cable directly from my laptop that the guide camera will be plugged in to? Or is the "GPUSB" from Shoestring a must?
If the mount and scope are set up properly the guide scope does not need to be particularly well aligned to the main scope, as evidenced by the fact that in an off-axis guiding system the guide star is necessarily quite removed from the main image area. You do need to be sure that the scope is orthogonal to the mount's RA axis. If not, over a long exposure (and especially close to the celestial poles) you will get field rotation. And since this rotation will be around the guide star, not the main image, it is helpful to get the guide star as close as possible to the main image.
Regarding the ST-4 port, it really has nothing to do with the camera. Cameras that are made specifically for guiding sometimes have an ST-4 port built into them just as a convenience, but it has no direct connection to the camera. The computer gets the image from the camera, analyzes it, and sends guiding corrections to whatever interface you select, including the possibility of one that happens to physically reside in the camera. On the other hand, your computer does not have an ST-4 port built into it. You could either use a GPUSB or, if you have an ASCOM link to your mount, select that as the interface for guiding and avoid ST-4 ports entirely.
I used GuideDog many years ago. Perhaps it has improved since then, but at that time it was very difficult to use compared to PHD Guiding. In particular, it had no calibration feature - you had to figure out the guiding parameters yourself. Of course, forcing yourself to do that might be instructive, but it might also completely turn you off on the whole idea of auto-guiding!
Hey guys, thanks very much for the information..........big help now all I need is some clear skies:)