If the OTA opening is too tight then I can understand why you would want to offset the secondary mirror away from the focuser -- I did include this reason on my list.
However, only stars that fall close to one edge of the frame might be impacted -- not the ones in the central vicinity and the opposite edge of the frame.
Quote:
Originally Posted by Shiraz
If you have a fast scope, it is worth doing a back projection exercise to see how much effect the secondary position/offset can have - it can be a bit scary when you see how far out a properly collimated fast scope can be and a lot of the woes of fast scope imaging can be traced back to this as the cause.
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I did it years ago with a laser and a holographic attachment. But you need to be careful. When the source of light is below the focal plane the reflected light off the primary mirror will diverge and gives you worse reading.
Jason