Comparing the brightness of the left zone with the right zone can be difficult, particularly when testing long focus mirrors. The longer the focal length the less contrast.
Some mirror makers prefer a cardboard mask to mask the zones.
To get these pictures with the knife edge near the radius of curvature of the mirror center, the 70 percent zone and the mirror edge we used an already completed and coated mirror and traced thought the mask onto the coating with a marker pen. Normally the mirror would be uncoated during these tests.
There are quite a few mirror testing programs on the internet. Programs such as 'six tests', usually take data for several mirror zones not including the mirror center and the mirror edge because these zone have been found difficult to read accurately. For more examples and links refer to the 'Surf Program' folder.
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