It's a common problem with ALL Chinese mirror diagonals in 2".Don thanks for this tip, but I'm curious, is this a common problem with this particular diagonal, or do you think these are "blems or seconds"? Thanks Bob
4) These star diagonals may have a misaligned mirror. To test that, if you don't know how to do it off the scope, make sure your SCT is well collimated with a 10mm eyepiece straight through (no diagonal in place).
In the northern hemisphere, Polaris makes a good collimation star. Then, place the star diagonal on the scope and check collimation again. If you see a lack of collimation, try loosening the setscrew on the diagonal and tipping it slightly. If this improves the star image, sometimes a thin aluminum shim between the star diagonal body and the tube into which it inserts will do the trick. In this way, the scope can be as well-collimated with and without the star diagonal. Remember to always bring the test star to the center of the visible field before testing collimation.
I don't think they're blems. I think they're surplus from what came standard on the larger SCT scopes.