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These eyepieces are available world-wide, under at least 8 different labels.
They work fairly well above f/7, but have ever-increasing amounts of edge-of-field astigmatism under that.
By f/5, they are barely acceptable.
In contrast, there are many 60° eyepieces in the same general price range that will give much better images.
And there are 70-82° eyepieces for not that much more that will dramatically outperform them in an f/5 dob.
My last use of these "generic" widefields (~66°) was in an f/12.75 Maksutov, and, in that, they worked quite well.
So, caveat emptor. Base your decision on the f/ratio of your scope and on realistic expectations.
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