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Matt,
The OAG "picks" off a portion of the image going to the CCD with a tiny mirror, at the edge of the FOV. Finding finding a star can be a real pain in the butt. IMO its important the OAG can be easily rotated to find a star, or it is used with an instrument rotator. The difficulty in finding a star (and the SBIG internal guide, same problem) is what made me make external guide work. On the other hand, a perfectionist would not consider this downside a hassle, in the pursuit of the best guiding possible with very long exposures.
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