Tony,
Like everyone has said, the imaging and guide scopes do not have to be perfectly aligned, but they should be pretty close. I use the crosshairs in both PHD2 and Sequence Generator to get as close to center as I can on a bright star without over doing it. If the centers are too far off you introduce cone error and over the course of a night on one target, the guide scope will stay on a star, but your image frames will start to travel around it in a small circle. Then when you go to stack your light frames you will see they will not overlay as well, and you will have wasted margins you will need to crop off, reducing the total good image area you might have had.
Just as important is good polar alignment. Even if you exactly center both scopes, the imaged frames will drift off target in one direction (declination), and then the other way after the meridian flip, when PA is bad. Then again you will end up cropping off bad margin areas.?
If you are imaging a target that has a lot of empty space to the edge, these factors are not as important. But if you have something large that just fits onto the sensor, then you could be losing part of your intended target. But with good centering and good PA, the light frames will pretty closely overlaid and you are getting the most from your given sensor size. And we all like bigger sensors :).
Best regards,
John
