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The slightly elongated stars at the corners are due to field rotation. With a slight misalignment betweeen guidescope and imaging scope after twenty minutes exposure there is a bit of field rotation. This is even more apparent with a full frame. This can also be caused by the guidecam not being perfectly aligned with the rotation axes when using guidedog.The Astro Physics focal reducer flattens the field of the 100ED almost perfectly.
Below is a 100% crop (bottom right corner) from a short exposure of the same field.
Bert
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