This is how the mirror looks like when seen through the testing device. Each deviation from the perfect shape is dramatically enlarged axially.
The image was taken in an early stage of polishing; the surface is relatively rough. Actually the central 'hump' is a deviation of only few 1/10,000 mm!
The mirror is ready polished as soon as the testing image is seen completely flat without any structure - this sometimes needs a lot of patience - especially with large mirrors.
The flat testing image indicates, that the mirror's surface is now a perfect spheroid. As a last step it is necessary to change this spheroid to a paraboloid - only then a 'Newton' delivers really optimal results!