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It doesn't have to be the best SNR. PHD is pretty forgiving.
Absolutely pinpoint stars (e.g. single pixels) are not good. Apart from anything else, you might be guiding on a hot pixel.
Craig recommends slightly defocusing so your guide star covers a few pixels, for instance 2x2 or 3x3. This reduces any effect that "seeing" has on the guide star.
For an idea of the size it can cope with, PHD will guide on Mars through an Orion ST80 with an Atik 16ic camera.
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