This is a basic topic for light weight mirrors that needs to be addressed and is often misunderstood and used to deride light weight sandwich type mirrors. Fundamentally, before considering print-through, the first things to consider are the overall stiffness of a mirror and proper mounting. These two factors are considerably more significant than print-through of the optical surface. The Hextek design offers the best overall stiffness to weight ratio of any light weight concept. If the entire glass structure “flops” around in different orientations due to gravity, or is supported improperly has far more effect on light loss compared to print-through.
What exactly is print-through? Print-through or quilting on a mirror surface is a localized slope error with the peak of the slope centered over cells that make up the light weight core structure between face plates. Print-through is introduced during the optical fabrication process of grinding and/or polishing. The basic configuration of a cellulare structure is shown below (left diagram). As the lap moves across the substrate surface, downward pressure is applied by the lap to optical surface (center diagram). The ribs provide a rigid support for the pressure but the unsupported cell center of the face plate bends or flexes (the degree of bending is dependent on face plate thickness and pressure), reducing the material removal over that area. The result is a hill on the face plate centered on a cell (right diagram). The implications on mirror performance can be significant in the form of light scatter and degraded image quality. When viewed interferometrically or even a star test, an obvious pattern of bumps can be seen centered on each cell.
The effects of print-through on optical performance can be significant, so minimizing the cause is very important. The are a variety of methods to reduce print-through but by far the most practical and affective is to reduce polishing pressures. Other methods include; pressure compensation to offset lap pressures (adds complexity to the optical finishing system) and fabricating sub-ribs into blank design to stiffen the unsupported face plate span. We prefer low polishing pressures of about 0.2 psi to avoid print through. Low pressure polishing does add to fabrication time but the resulting surface is superior.