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Taking the minor axis of 57mm on a 250mm scope you get a 22.8% obstruction by diameter. This is in the ballpark of the generally accepted obstructions but toward the high end. Generally the rule of thumb is between 17-20% with obstructions up to 25% acceptable.
If you want to go for the low end you need a really good flat mirror that is good to the edge so you don't get distortion. That is why I spent the extra on a Discovery secondary for considerable more money than a GS one.
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