The same procedure was discribed in a book for ATMs written in Russian which was published in Moscow about a decade ago. I also want to mention what as I know opticans don't use such channels on the pitch lap at optical workshops. They just make random scratches on the pitch lap. It's more easily to make and takes only a few seconds. As disadvatage you need to remake ones more often then deep channels, but it also allows very easily to introduce some zonal changing in pitch lap pattern if it would be nessesary for correction mirror's surface.
Best regards, Alexander Yascovich yas@hea.iki.rssi.ru