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Hi all,
Well to be honest I dont use diagonals because I like newtonains and have no need for one. However, I can tell you this. All the physics suggests that a top quality mirror diagonal "must" outperform a top quality prism diagonal. Less air to glass surfaces, less affected by temperature change and no possibility of introduced chromatic aberration through refraction. Further, it is infinitely easier to produce an optically excellent flat mirror, than it is an optically excellent refracting prism.
To the practicalities of it all.
The fact that a cheap Taiwanese mirror diagonal performs at the same level as an expensive Japanese prism diagonal has to support the theory that it's easier to get a mirror diagonal right.
The fact that several people can't pick any difference between the two tells you that while the theory suggests there should be differences, its likely they are pretty small in the field.
Cheers,
John B
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