This means that you are looking at a very narrow zone on the mirror, a fraction of an inch, because the zone that corresponds to a measurement +-0.001" will be very near by indeed. So, this means that instead of the typical 3-10 zones, you will need to measure 100 zones. If you do not sample at a rate consistent with the width of individual measurements, then you cannot guarantee the veracity of your results. Sampling theory insists on being taken seriously!
So, anyone who claims 0.001 ability and then does not take 100 zonal measurements across the mirror face, cannot say anything about the mirror curve anyhow.
I feel good when I can do 0.005". You all know I've done maybe 100 mirrors over 25 years and conducted numerous mirror making classes, so you can judge my 0.005" statement accordingly. Claims of 0.001" sound absolutely fantastic, and uncomprehensible to me.
Mel Bartels