Thanks guys! You’re right the knife position is not optimal. I did manage to get a somewhat optimal knife position (however, without a third hand, I didn’t manage to photograph that) and then the light marks furthest from the knife are at about 28 degrees on each side. The large patch of light at the right remains there.
An acquaintance of mine also mailed Catra (makers of the goniometer) and got a response from them. Nice guys! (And of course their goniometer can do a large convex knife 🙂 .) This is their response:
Extent of LH edge polish 21 to 23° convex Extent of RH edge polish 21 to 31°
The actual angle is read along the mid position line. Note that the blade is not centrally located it appears to be offset by 2-3° to the RH side, so the actual readings need to be adjusted by this minus on the RH and plus on the LH The result of this is typical of edge polished blades. Whilst the primary grind was intended it appears to be around 32° ( 12 +20) at the edge, the subsequent polishing has actually taken the final edge angle to 54° (23+31) included angle. During edge polishing high sharpness levels can be maintained if the polishing angle is the same or slightly more than the primary grind angle.
I don’t understand everything they say (21 to 23° and 21 to 31° ?), but I am happy they conclude the final edge angle should be about 54° inclusive. That is almost the 28 degrees per edge I inferred.
